DeGoogle Yourself

Synology Drive does what you want, if you bought a Synology NAS. Look for the Synology Drive or DS Drive app for each of your operating systems.

Whatever you're using, I find it helpful so to setup Samba shares, since most operating systems can talk to it.

Some command line utilities that you may wish to schedule, if not using a sync app:

• I've heard great things about KDE Connect, which supports a bunch of platforms.

• On Windows: robocopy.exe

• On Linux: rsync

• On Mac: rsync

• On Android: I don't have a non-Synology favorite, right now. KDE Connect looks promising.

• On iPhone: I've heard of these "eyes" phones. People seem to like them...

Edit: I see you have Syncthing. Disregard the above.

For NAS to NAS backups, I do prefer RSync.

Borg

Use something like rclone (that can use file change deltas) when you just need to copy files to remotes and feel free to combine it with an incremental backup solutions like Borg or restic or some custom rsync scripting. Example: keep a Borg repository of your laptop or emails or whatever with whatever retention policy you want. Then copy your repository anywhere with rclone or similar.

Syncthing is excellent for phone sync.
What I did was have it running on a system in the network of the nas, mount the nas on that system, and place the backups folder in the nas.

If you have a system that reliably runs, or can get syncthing running on the nas, I recommend doing that.
Synology has docker iirc, there aught to be a syncthing container.
Else, slapping a pi zero into the nas' network should do the trick and be fully independent of what the nas is.

My phone syncs to my nas with auto sync via SFTP when at home.

I have an rclone job on another system that then syncs the nas to rsync.net.

There's borg in that mix too but out of scope for your question.

I've been using Duplicacy and like it. Everything backs up to my server via Seafile (files) and Immich (photos). Then Duplicacy has a couple of B2 buckets it backs everything up to. Seems to work pretty well so far.

I've tried numerous terminal based backups like restic and Borg, but my newb brain still needs a GUI I guess. I've seen various UIs for those last 2 but haven't tried them yet.

Duplicati 2

I'd recommend spinning up a backrest docker container on your main NAS, which you can then use to backup to all kinds of sources. You could then for example expose a WebDav share on your second NAS, and setup automatic backups for there.

Even though this is the DeGoogling sub, you could also use Google Drive or OneDrive as a backup source, as backrest/restic fully encrypts all backups.