I use syncthing for backups including some phone files, but I’m not sure this would be good.
Syncthing devs clearly don’t want this app used as a sync-and-archive tool so all phones would have all copies and any phone can permanently delete any file. I wouldn’t trust that.
(Yes, there is a roundabout way, I do it too, but it is prone to errors and sync issues)
I second immich and backup. immich can archive as you want, and Syncthing can make backups of files.
It’s changing settings to only allow one way sync and disable deletion. The sync folder basically becomes an automatic archive destination.
They are soooo close to having this cool tool, but many feature requests have been shot down because it’s not a true sync. I get it, but it sucks too.