I have 4 examples of different stories i made with different audiences in mind. Sometimes readers overlap but is not always the case. There are always some readers that may be outside this audience, but the ones shared here are the most prominent ones.

1-Rocky the Rock is a shonen parody about an inanimate rock being an overpowered shonen protagonist.

It appeals to people who love anime and people who love absurd humor, the demographic on this one is peripheric, because it is enjoyed by people around 13-40 years old, as long as they share these interests.

2-Superduperdoopyclub is a lighthearted surreal comedic slice of life about a weird club that may or may not be a cult.

The simplistic art style and surreal lighthearted tone appeals to kids, but also to some adults.

3-Legends of Camelot: is a subversive retelling of King Arthur's story with a more detailed art style. (had to hire an artist and a colorist for this one).

It appeals to people who love medieval fantasy stories or games, and people who prefer a more occidental art style.

4-The princess villainess stares at a blank wall and does absolutely started as an inner joke about the latest Tapas trend
of overly descriptive names involving royalty and villainesses.

It appeals to people who love absurd humor and people who are part of this inner joke in Tapas.

The content defines the demographic, not the other way around....so who is your audience will depend a lot of the kind of content you want to create.

as a general rule, complex stories will appeal to older audiences and simpler ones will appeal to younger readers.

Some stories may be too dark for younger audiences, but there's always some people who are looking for edgier stuff.

Lighthearted stories also have a strong audience, sometimes people just want a good laugh or some form of escapism

Make a story you enjoy, so you can have the drive to work on it.

All my comics and novels are always aimed to please my 13-year-old self...so I guess my target demographic is all the edgy teens and tweens out there, who want to read fun yet serious stories that have young people in mind but don't talk down to them. If you like YA media that isn't just a thinly-veiled vehicle for Love Triangle #264, I gotchu fam~

And I think that's the way you should go about deciding on a target demographic: looking at what you like to write, and deciding which audience probably wants it the most.

Going the other way usually does more harm than good: for example, occasionally, I decide I want to write a story that's 'more adult', that will be received as a serious story by older readers. So I try to create something expressly for that demographic...and it usually fails. I'm 0 for 2 at this point. ^^;

The same goes for when I want to try to create a story for younger kids...instead of trying to tailor the way you write to fit a certain niche, it's better to just let the way you naturally write fall into whatever niche is most appropriate.
Then, 'aiming' will consist only of trying to spotlight certain things that you already like and know how to do. When I try to aim for the older half of the YA demographic, I just turn the dial up on my usual 'romantic antics' and the blood and violence I love. And when I try to aim for the younger half, I use more of the cartoon humor and melodrama that I've been perfecting for years. ^^ It's all stuff I already do; I just adjust the recipe.

To tell you the truth: I rarely think about what my target audience are when I create my story.

I make the story the way I want, however I do try to make my story more simple & universal to draw in readers. The story has teenage protagonist and it's a lot like shounen manga, but tells in a art-house movie kind of way. It's not pulpy that's for sure and I hope my readers realize that.

My comic does not contain clear romance so it doesn't limit out readers who only like straight romance or BL etc. Also doesn't have very strong sense of male/female gaze (except some comical characters) Also I don't want to focus on character representations, just be story oriented. (aha this may limited out some readers too)

I do try to be as approachable to most readers as possible but in the context that they have to be able to stomach my anthro designs & the story (and also my not so good English, which could be a welcoming thing for international readers ) Also when future chapters where things go dark, I may lose some readers too. that's understandable and I'll be okay with it.

Most of all one just have to show off the uniqueness of your work (within rules), and do so confidently. There will be audience who like your stuff.

I think the main thing is that you now understand trying to make something for everyone is really tough and while not impossible to do solo, probably requires an entire team of people.

That said, I personally believe that the notion of picking a set demographic for your work is something that matters only if you're set on marketing it and/or selling it to a big publisher. Because in that case, the stats and details really matter.

BUT
If you're only interested in making a comic for yourself and you're not hanging any monetary expectations on it, go for broke and make the comic of your dreams.
In that case write the story you really want to tell and let it find its own audience over time.

Edit:
I just realized that I didn't say how I chose my demographic for Dragon Sparking. I basically make what I liked to read when I was 12-13. So pretty much Middle Grade Action. Even though in actuality, I don't have any stats on the ages that read my comic

I sort of keep my series in the PG-13 range. I am not really interested in writing 18+ stuff. I do have one series which is All Ages and I guess the struggle is making sure you write something that no one would find offensive but a kid would still enjoy reading.

My series Sugar Land was also suppose to be All Ages but I one point I just gave up. I heard someone set out a challenge to comic creators to create more LGBT stories that preteens could read without having to deal with adult jokes. That is what I originally set out to do for that series but I definitely failed.

That's the theory yeah. But there's still plenty of other factors into getting eyes on your work, just making it isn't a guarantee.

But the real question is do you even want to get back in the game yet? Or do you still need some time to process and relax?
Whether or not writing down ideas is a good exercise depends on you, really. For me, I already know what I want to write, putting it down on paper doesn't help me at all.

Then, whatever project you do, do it for yourself, on your own terms. Make this your space and your time.

If you saw my comic that is based on the Fantasy genre called Mukhtar, you would think I am targeting some specific audience but nope. I made that comic out of passion. I even planning to make another passionate comic in the Sci-fi genre. I never have any target audience, it's for everyone to read and it is up to them whether they liked it or not.

The target audience is ME

Okay but fr I rarely think about target audiences when I write-- I write what I would enjoy reading or find interesting. I guess in that case my target audience ended up being people who find the same or similar things things interesting: psychological horror, darker stories and villains with complicated relationships with the heroes.

For me it way relatively easy I guess, since my stories have pretty clear inspirations in the sense that I can clearly identify 'oh, I want to make something with XYZ vibes'. So my target audience would be the same as the target audience of my inspirations

One thing though is I don't think it's useful to think of target audience in terms of age/gender etc. Rather, think genres and special interests. If someone posted a panel of your comic onto a subreddit, what community is it most likely to be?

Don't forget you yourself are also part of a demographic. What communities do you frequent? What tropes/inside jokes are they familiar with? You probably have a few insider perspectives already; tap into that!

I'm kind of bad at aiming for a specific sort of audience. There are a few things I've been trying recently to really narrow things down because I would like to pitch some stuff at some point, and I also of course work for a company that makes edutainment for kids, so I've had to learn to deliberately write for different age groups.

1. Look up research on what different audiences react to. One of the things we get at work are all these research presentations and things about what colours age groups react to, how they play, how and when they read and watch, how they identify with characters, how many things they're a fan of, their vocabulary level etc. Actually a really good resource when writing for kids is to look up the school curriculum or syllabus for that age group to find out what the base level expectation of knowledge and vocabulary is for them.

2. Comp titles. A comp title (comparison title) is a series you can point to and say "it's kinda like this" when pitching to publishers. A comp title should be in the same medium and aimed at the same audience as the thing you want to make (so a mistake I made on Errant was I went in with Netflix She-Ra and Steven Universe as comp titles even though they're animated and webcomics are a different medium with different demographics and trends), they need to be within the last 10 years, and they need to have done pretty well (A publisher isn't going to be put at ease if you say "it's like this comic that was a total flop."). So if I wanted to pitch Errant as a Tapas Original now, I'd have Pandora's Devils and Hard Lacquer as my comp titles (because most people who subscribe to Errant have them in their libraries and they're both popular Tapas Originals), and for a print comic I'm planning that's for kids, I have things like Witch Boy and Amulet: The Stonekeeper. A good start is asking friends for recommendations, seeing what people who follow your work are into and just reading a bunch of comics and being like "Aha! This one is my vibe!"

Like @smokesalty wrote,
my target audience is usually me.
But that depends on what I want to achieve, I´m taking part in a comic contest right now,
the comic will be printed when we win and be eventually read by kids, that means the target
audience can´t be me anymore and I will make it kid friendly

My target audience is pretty niche. (Anyone who likes sci-fi horror with industrial themes, body horror, etc. and also like manga art.) I didn't really have a set plan but I just post my work and follow similar creators. It's helped build an audience over the years.

My target demographic would be older teens and young adults who are into Pastel Goth, dark and edgy stuff. My series "Stacy The Demon Killer" is a parody and mix between Buffy the vampire slayer and devil may cry. It gets pretty violent with all the blood,gore, and swearing, so it's definitely not for everyone.

My two series is targeted towards battle shonen and seinan fans, though they're both novels. One I made Crusaded Defender is a series that I hoped HxH Fans would like and Agents of Salvation is a series that I believe would gain the attention of D.gray-man/monster fans so these are not flat out plagiarized carbon copies, there's just inspiration from those series mixed in. I also was targeting an audience who was in late teens/early adults range, both series are action, with Agents being horror and tragedy, and CD being Adventure. In the end I hope that kind of audience is present on this platform.