Syncthing is tricky to get working properly all the time + it drains quite a bit of your mobile devices battery life in my experience.
However a co-worker recently shared this interesting FOSS project, EmulatorJS, it's self host-able RetroArch, playable from your browser:
https://github.com/EmulatorJS/EmulatorJS
I haven't played around with it myself yet, but it sparked my interest, because it would mean you don't need to sync saves between devices, but still can continue playing from your save from any device which can open a browser.