I understand that fear of going too far one way or another in terms of tone. But honestly, for my comic, I feel like I'm dancing that line.
I'm aiming for a general PG rating, given the "Don Bluth" vibes as someone described it, but my story also has dark humor and serious undertones that would suit a more mature audience (13+). What would make the darker stuff more apparent, to me, is the intelligence/maturity/exposure level of the audience.
I would put up content warnings on episodes that feel a bit too - harsh, but it's also a good idea to hint at whatever "iffy" content is in your comic in the first few episodes - with a warning or rating, just to dissuade the "sensitive". Like GoT, for example. S#x scene within, what, 10 minutes? Acts as a filter of sorts. But don't overdo the warnings on the small stuff, like light/censored cursing or mild blood (may have to look at examples of what others consider mild/light). The last thing any adolescent wants is to be treated like an innocent, sensitive baby, like the Internet wants us to treat them.
Just be honest and upfront in your adverts, and do whatever fits the story. Despite any age rating, I'd personally wouldn't mind a balance between light and dark, funny and serious, in the overall arcs. I'd even say it'd be worth it to go hard both ways, if it suits the story. (My story's kinda dramatic, but it'll get a bit more comedic in the coming months, just to achieve a balance. Whether it works will depend on views and comments, it's an experiment of sorts.)
A lot of stories tend to shift tone, albeit gradually, through its course, as do we (especially moody teens, lol) so don't worry too much about it. If you want to explore something dark, put up a warning and move on with the story. Worse case, they can skip that part, if formatted right.
The demographics will sort of sort themselves out, if that makes any sense. In other words, your "target" audience will find you, if you present the right stuff and cover as much ground as possible.
For the record, I had assumed my "target" audience would be farmers and poultry workers, lol! But I can't please everyone all the time, so I just tell it (my story) like it's supposed to be. Whoever likes it, keeps on reading/ Whoever dislikes it, well, I'm satisfied that I wasted their precious time, lol!
Sorry this was way longer than I wanted it to be. Coffee's kicked in!