I bought a book about writing a series (one of the few available) and the author talked a lot about her past issues changing publishers mid-series, etc., and didn't share a lot of information that was useful.
For me with my series, a big shift was when I got serious about a series bible where I noted everything about the characters, settings, etc., so I could go into detail on side characters and keep them consistent.
But I'm in no position to give advice, because my sell-through from book 1 to 2 and so forth isn't that amazing. Readers who stick become superfans, but I always lose a lot of people after my first in series.
The indie authors I've observed who have amazing sell-through from books 1-2 are actually the ones who split what I'd call one story into a trilogy. i.e. book 1 doesn't have a satisfying conclusion and doesn't stand alone. That will go against the advice most would give, about giving readers a complete book 1. Many of the indies who are selling buttloads of a trilogy just ignore that and split a story. FWIW.