@maddy Resisting the urge to post solutions because (1) there will probably be plenty of reply folks proposing them and (2) you've undoubtedly thought about this much, much, much, much more than I have. Instead: sympathy on this set of problems and tradeoffs from a dev in other spaces.
What I will say: I challenge the fundamental notion of what makes the game work in a theoretical sequel. Folks that want their workflows to work or mod will play... Celeste. What do you want to do with a 2.
@maddy IMO, I thought Celeste 64 had much more to say about what a 2 would be like than Celeste did. Y'all came together to build something small scale again that did what you wanted it to, with all its rough edges and warts on the dev side that we undoubtedly don't see as players, and gave something familiar but not entirely the same that spoke its own language. And I think y'all had fun with it?