I have a local server that I store all my personal documents on. It's backed up to a 2nd internal drive, btrfs, which then takes a snapshot.

It's also backed up to a cloud server with borg, which is encrypted and doesn't trust the endpoint.

Aside from that, I keep all the paper copies of forms I receive in a filing cabinet. In January each year, I put the last year on a folder and just throw all my paperwork in there as it arrives. I have folders going back to 1999, the year I was married.