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:
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I’ve heard great things about KDE Connect, which supports a bunch of platforms.
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On Windows: robocopy.exe
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On Linux: rsync
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On Mac: rsync
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On Android: I don’t have a non-Synology favorite, right now. KDE Connect looks promising.
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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.
I don’t have an answer. This issue is why I left Proton mail. I’m waiting for a built in (or reasonably automatic) solution before migrating my email back to Proton.
Thank you for asking here. I’ll be reading along in case someone has something that’s not too burdensome.