i do backups of my home folder with Vorta, tha uses borg in the backend. I never tried restic, but borg is the first incremental backup utility i tried that doesnt increase the backup size when i move or rename a file. I was using backintime before to backup 500gb on a 750gb drive and if I moved 300gb to a different folder, it would try to copy those 300gb again onto the backup drive and fail for lack of storage, while borg handles it beautifully.
as an offsite solution, i use syncthing to mirror my files to a pc at my fathers house that is turned on just once in a while to save power and disc longevity.
the matrix room may be public, but when you choose someone to talk to via dm, at that point encryption matters. Of course, you shouldnt really trust that random person from the matrix room, but it being an encrypted chat reduces the risk somewhat.