I tried out KDE once as a cross-platform version of airdrop. It worked but the UI was pretty buggy.

Syncthing is great. I switched from Android to iPhone recently, and I still have Syncthing but I've slowly stopped using it because it just doesn't work as well on iPhone. It doesn't sync automatically in the background, and plus the best feature was how it synced my photos folder and it can't do that on iPhone because of some stupid Apple reason - of course Apple has their own way to do that where you pay them to store it in iCloud so that's probably why.