If you go the Syncthing-Route anyway, take a look at Softmaker Office [1], it's an almost-drop-in replacement for MS Office and would set you back 50 EUR/year for 5 devices.

We evaluated it for our migration away from MS software and would have gone with it, but it lacks an office server for Nextcloud integration.

[1] https://www.softmaker.com/en/products/softmaker-office