The phone here basically does IMAP (which is sync I suppose) and gets plugged into the computer and stuff copied around as required manually, which turns out to be rarely as it's not the primary device!

I haven't decided yet :)

More seriously, I am mostly working like this now. I've had at least some data loss or reliability from every single sync solution I've tried so am practicing avoidance where possible.

I really want something to work but I can't find anything that does and I've tried all major ecosystems and syncthing etc.

its something, lets move along quietly and hope they dont notice...

also not sure why so many have a love affair with syncthing, id never heard of it but more diverse software in the world is a good thing imho. the more wheels reinvented the better, its fun!