This is not to counter your point (I agree they should probably offer a client to sync files on Linux), but rclone recently added (beta) support for proton drive, so you might want to check that out if you're still using it.
I'm not sure about pulseaudio but snapcast is time synced.
This is not to counter your point (I agree they should probably offer a client to sync files on Linux), but rclone recently added (beta) support for proton drive, so you might want to check that out if you're still using it.