I'm not sure if I'd agree that you need to flesh out ALL the characters more than you need to. The main characters for sure, but I think corollary has the right idea.
corollary
Don't get caught up in trying to make every single character "well thought out." Not every character has to be round. A combination of round and flat characters is necessary to a story, mostly because if every single character was round, you would be spending so much time developing the characters rather than the plot that the story would be lost. You have to think, "how does this character contribute to the plot?" and if they don't contribute anything, edit them out or change their role. And if you have an action you need, but there isn't a character that already serves that purpose, but you only need the action once, don't be afraid to have a flat/"throw away" character. And if we're redefining "well thought out" as "applicable to plot," well, you tell us.
I think the most important thing is to have fun with your characters. Be passionate about getting to know them better. If you're feeling stressed or pressured to make every character super realistic and unique, you may lose interest or come out with really awkward, needlessly detailed characters.
If you're adventurous, I'd suggest writing the story before you even know them very well. You can learn about the character as you write about them; the reader will be learning about them through the course of the story, so why not you?
You can always go back and edit things you don't like. Even if it's a webcomic. It's your webcomic, so no one's gonna sew you for retconning it ):<