

Some very cheap phones with minutes included, here:
https://www.qvc.com/electronics/phones/tracfone/_/N-mlt0Z1z1393y/c.html
OTOH they can probably be traced back to you since you have to order them with your presumably real name. You can buy phones and prepaid cards anonymously in phone stores but it will cost more.
Maybe nextcloud? IDK I just use Borg. But Nextcloud allows that type of syncing that you describe, I think. I run a small nextcloud server for other purposes and don’t use that feature.