This is where I feel glad that because of a fairly long history making comics, a lot of my beginner mistakes are in the past. 
Plus of course, knowing a lot of comics people, I could ask for advice before starting. That said, it kind of is the first webcomic I've made in ages. I took a 9 year gap between Fan Dan Go and Errant, and only made a few print comics and stuff in that time, mostly working in illustration and games, and I think it shows.
Things like working out how to space out the panels for the pseudo-long-scroll format and use panel space, a lot of which caused more issues converting uploads for Webtoon and then print and aren't that obvious in the comic itself, like relying a bit too much on the increased gutter space to fit in the text because the size of the text my partner recommended was bigger than expected.
Another issue is that due to excitement and enthusiasm, while I did do style tests, I didn't really do a lot of them, and looking at early pages, I think it is kind of visible that I'd been through a period during which manga style art was terribly unfashionable (I know it sounds weird, but at the height of Tumblr, there was this period of like maybe 5-8 years between the Tokyopop crash and before the rise of longscroll Webtoon style comics where any kind of artstyle like that was "embarrassing" "weaboo" etc.) and mostly been working in this more "indie cartoons" looking stuff. That said, I sort of like how by a happy accident, the story starts off looking almost like an indie mystery comic and then slides more manga looking as the story gets more like an action manga!
The big one in a way is the choice of comic itself. I made Errant because I was passionate about bringing back what I think of as some of the best characters I've ever come up with and a pretty sweet magic system. I'm not exactly disappointed with Errant's performance, but realistically I have to acknowledge that it's not the most optimal comic I could have made to build an audience in modern webcomics.
The big point of contention is the prologue. It was probably a bad idea to start a webcomic in the modern space where spelling out your premise in the first few updates is standard, with a very long prologue where the characters aren't the sexy 24 year olds they will be for the rest of the story, but dorky little 14 year olds. After thinking it over from every angle, I think the prologue is the best possible way I could have told this story.... but then that raises the issue that "maybe this story wasn't the best suited story to the current webcomics environment".
I really love Errant, and all things considered, it's performed really well, but it's not really optimised for performance if the aim is making money, because I went in unaware that big webcomics these days tend to use narration so they can set up the premise quickly, skip to the plot catalyst as fast as possible to catch attention, then backfill in the "setup" part of the story after that as needed, and they also tend to be long-scroll and a more straightforward Romance or power fantasy type story. You can kind of tell I'm from an older period of webcomics in how Errant sort of slowly builds up from a light mystery and jokes into this more intense Action Fantasy story, adding in lots of wacky characters along the way. It's definitely more "Scary Go Round" or even "Homestuck" than "Second Life Ranker".
The things I did "wrong" kind of give the comic a very unique vibe, but do make me wonder if it can ever realistically make me a little pocket money on the side.
