I've been working on a quite complex React codebase for 5 years now - it's not the biggest but still not so small. I was there on day 1. The team that works on it now full time is ~15 devs + 10-15 doing some minor stuff from time to time.
We've started using hooks about 1 year after they introduced them. Before we were very deep into redux and redux-saga. Now most of our codebase uses only hooks for most of the things we did with redux before - there are still some redux but most major parts are migrated to using hooks and contexts. It is probably the best decision we made after switching to TS. Most of our codebase is still very easy to maintain. Last year I went on a paternity leave for 6 months and I was pleasantly surprised how easy it was to adapt back.
Edit: forgot to write about Elm. I like elm a lot -even though I don't agree with how it's managed-. I even organized a local elm meetup :) But I think while it is very good in theory, it's not too easy to scale especially if you plan on hiring more devs. Also the way effects and data is completely separated from the view is not very ideal for bigger projects. You either end up having to touch too many files for a single feature or have incredibly big files.