At last - finally got a satisfactory syncing arrangement working.
1. #Syncthing on main work laptop, with the syncs in send-only mode.
2. Syncthing on office #Diskstation in receive mode.
3. Syncthing on a DO #droplet locked down in #Runcloud, also in receive mode.
Approx 30Gb is being synced.
This will be coupled with weekly snapshot backups to the Diskstation using FTP.
So the only cost is the droplet, but DO affiliate credit will cover the next 18 months.