ZeroTier kept having random disconnects, long wait times until connection is established/settled, and desktop app weirdness/inconsistencies. Have these problems been fixed? (Last used it years ago).

Yes and no in my experience. The past year I have had some trouble but the Linux clients seem to be good again. Win11 is getting worse for me unfortunately.

Seems fixed. I'm using it from 2 or 3 years and it's very strong.

Mostly used to connect few Linux boxes behind NAT or double NAT without opening ports on router. Also running on macOS.