With Syncthing, you do not need to have every device in the same network. But
you are right, when syncing many devices, Syncthing might make you miss the
piece of mind of a server-client architecture.
On the Nextcloud client for Android, I agree. It is very sad, you cannot
easily configure a 2-way sync directory, and they do not fix it (at least for
the last years on F-Droid versions).
I have not tested it, but someone told me Seafile syncs better than Nextcloud
and its Android client is ok too. I have never tried it, if you do, please
let me know.
By the the way, if you really need a phone, have you ever considered to
abandon Lineage and try Replicant : ?
