I am frustratingly blind to today's alternate OSes. Where does a lay-consumers even start? Do I buy a used, but well supported (by alt OSes) phone? Which one would that be? Do I attempt to use my existing phone?
Recently I feel like alt OSes have become really mature. I have been using a de-Googled /e/OS on a Fairphone 3+ for 1.5 years and I don't have anything to complain about the system. Even the store now works very well.
So I'd say, for your next phone, choose something that is well supported by those alternative OSes (typically Fairphone is), so that you can try, and fallback to stock Android if that's not working for you.