I did (and still do) a lot of mistakes, but none is really worse than the others, so I don't really have regrets. I'm happy to have improved, but don't really see specific things I'd wish to have known beforehand.
The only thing I would definitely do different is to harass my (editor) spouse CONSIDERABLY MORE to make them edit my comic, in order to be able to publish in English rather than in Frenglish 
Now the first chapter is edited, but I'm slow, very slow to make the changes.