Is it necessary to be FOSS, or is free good enough? If cost is the only driver and the backup is local to a regular device, I can recommend SyncBack for simple backups on their free version. I bought Pro several years ago and there is no ongoing licensing fee. When they add certain services I would find useful, or as tech progresses, I’ll upgrade but there’s no ongoing cost and it runs reliably for me. It’s also very windows-centric in terms of its design and UI, which makes it relatively easy to use. Granted I wouldn’t give it to my mother to set up for herself, but it will run unattended once set up.