Syncthing + rsync. Syncthing mirrors. It can be uni or bi directional.

Rsync on top is great because you can then rsync the server side of the mirrors to another directory or file system, without --delete, and make a pile of files over time that's larger than the mirrors. For example, video, music, or memes etc can be collected over time and that collection can be greater than the syncthing mirrors / what's on the phone.

Next, you can use something like Mixplorer Silver to access the remote file system over sftp, so you have access to all of the files.