Hopefully I can give some advice as somebody who makes an action adventure type series that kind of has 3-5 main characters plus at least one antagonist, multiple arcs and political intrigue, plus supporting cast. I don't normally include a link to my own comic in answers, but I think it might be worthwhile for you to at least skim it to see how much plot and characterisation I've crammed into probably about 300 or so pages so far:
So, here we go...
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Get efficient with your writing. You need to find the most efficient way to introduce a character, explain what their deal is and get their character development rolling. You won't have time to do a character's arc where nothing else is going on, and then also do a big story arc. I'm always looking for ways I can fold character introductions or development into establishing or moving plotlines, or ways an action sequence can also fill in characterisation.I often write out a script and then similar to @TheLemmaLlama I start storyboarding and go "UGH having them stand around talking to get all this stuff across is going to be so boring to draw! (and for the audience to look at), so you frequently get characters talking while walking to a place, or talking while in a fight, or talking briefly, only to be interrupted suddenly by a character intro or a monster barrelling in there.
My other favourite technique: The first page a character appears on, the first thing they say and do, should tell the audience everything important about that character. Whatever any character first says or does should ideally be so typical of them and make such a strong impression that you don't need to spend ages faffing about with people explaining what their deal is.
Another trick I use is paired character development, where my cast is made up of characters who all contrast each other when paired up, so putting any pair of them together will get you an interesting contrast that tells the reader a lot about both of the characters by showing how differently they react to the same thing, and puts them into tricky situations (perhaps pushing one of them outside their comfort zone) that will force one or both to develop. It basically allows me to double up character development, like Chapter 1 is both about Subo learning to overcome his low self esteem and take responsibility, and Rekki learning that her isolation hasn't made her strong and she should give other people a chance at the same time.
It's not all about you; it's about your readers. If your updates aren't entertaining, your comic won't get readers, so unless you're only making it for yourself, you need to think about what you're providing the people who read week-to-week. Every update needs to have a plot reveal, a moment of high drama, spectacle or action, something sexy or a joke, so pace around that first.
Most readers are more interested in the emotional journey your protagonist goes through than the complexities of worldbuilding, your magic system or politics. You can always limit pagecount down by just focusing on what your protagonist is aware of about your world, and leaving out any political or magic system stuff that doesn't affect one of your main characters. It doesn't matter. Make it an extra if you like, but most readers will just go "oh, that's neat" on reading these details in a one-page extra, and would be sat there like "huuuuurgghhh..." if you spent 6 pages explaining it all through a character.
Some characters need more screentime for development and explaining their deal, don't plot out assuming they all will need just as much. Errant kind of has five main characters; the ones on the cover, but a character like Rocket is meant to be kind of mysterious and she won't really develop much because she's already like this ancient being. Ordinary bloke Subo, meanwhile, doesn't need a big personal arc or vision quest, because just being there having to look after everyone and have that responsibility IS his character development. This allows more time to be spent on the other three and their more complex political stories. If I was adapting the series into a cartoon, yeah, it'd be great to use all the extra time I'd have with multiple 10 hour series to add in episodes specifically focused on each character... but I don't have that, I have me making 2 pages a week! So rather than focus episodes, characters might get more like... 6 pages of a chapter specifically focused on them!
It goes without saying, but keep everything just about your main characters. Don't do a My Hero Academia and decide to make an entire arc about the villains; you don't have time. Don't do a Bleach and introduce like 10 new villains who all have a complex personality and backstory every arc. Your release schedule is a couple of pages a week, tops, not twenty, and ideally you want to be finished in under 10 years; webcomics can rarely be milked as long as a print franchise.
Anyway, I hope that's all at least somewhat helpful. That's how I keep things moving along with this kind of project. I'm probably over a third of the total way through the comic right now.