Syncthing for synchronization between devices, then rsync to a couple of flash drives for cold backup.

The setup is so easy, you don’t even need a server for this, just your ordinary PC.

There is no more paranoid solution than owning your files on the devices you own as well, so you never have to trust the third party.

Additionally, clunky all-in-one self-hosted solutions like Nextcloud bind you to their ecosystem, so it may be hard for you to switch to another one.

Just a plain file backup to your own storage will always be a savior for your lost data, regardless of third party apps or infrastructure.