Rightly or not, I'm always a little cautious about synchronisation - I worry about stuff getting overwritten, and this has bitten me in the past. But I've not used syncthing so I'll take a look at that.

Movies: I remember when Microsoft dropped PlaysForSure, and i think that over a long enough timescale "my" google content will go the same way. No easy solutions there, unless i go back to physical media. Ideally there would be a backup solution like Kalibre for Kindle ebooks