Brickfrog

Eh, sure OP could do that. Does seem a bit over the top for OP to pursue the most complicated backup solution possible :D Maybe as a strange experiment to see how it goes, not as a trusted backup solution. (like you said not for critical data)

IPFS would also require more bandwidth vs just about any other solution since it has to constantly talk to other IPFS nodes. And more finicky, last I used IPFS the client would run into memory leaks and other weirdness requiring restarts every now and then (hopefully it’s more stable for long-term runs nowadays).

Similar but no, Syncthing does not use bittorrent or the bittorrent protocol.

Though if you’re curious Resilo Sync (formerly Bittorrent Sync) is similar to Syncthing and does use bittorrent.

Wouldn’t be a good solution, you’re hoping that other users are going to volunteer to pin (aka store and seed) your personal backup data for you.

Using IPFS for personal backups is exactly the same as creating a torrent with your backup data - With both it would be unlikely that your personal backup data will actually exist anywhere beyond your own data storage, no one’s going to freely volunteer to store your backups for you.