ojeez, quite a lot lol. Here's a random list:
1.) something that seems obvious in hindsight but that a lot of people (myself included) miss at first is to design panel compositions with the speech bubbles in mind. I know how much people plan ahead varies a lot from person to person (from having the whole story planned on on one extreme to making up each page/episode on the spot as you go) but regardless of which style suits you, there's no excuse not to at least know the dialogue for a page/episode at the thumbnailing phase, such that you can figure out how much text there will be and where it can fit.
Granted I feel that vertical scroll comics have some more flexibility with this because worst case they can just shift the bubbles up or down into the space between panels and just add a little more if they need it. This is harder to accomplish in traditional pages, as any bubble that breaks out of the panel will likely be covering up a different one and so you have to make a sacrifice somewhere xD
2.) Comics help all art skills develop at a rapid pace I've found, as you're drawing a lot of different types of things and frequently, but I've personally made the largest strides in understanding perspective better and leveling up my backgrounds by doing comics. It's fun to see that improvement episode to episode and especially comic to comic~
3.) How to structure my free time more rigorously. I've always been into drawing since I was young, but even when I started taking it more seriously as a hobby, I was still rather undisciplined about it. Starting comics and especially with a page-a-week series helped me block out several times per week to sit down and just get some work done, which is great for finishing projects faster lol. But it's also applicable to other hobbies/just general time management discipline as well.