How about good old offsite HDDs, tapes, etc. I guess it depends on the target. If it’s family photos, you probably want something like this – after all, if you get hit by a bus and stop paying the hosting bill for a couple of months, all that stuff could be gone.

Variations on the scheme include rotating media into safe deposit boxes at a bank, etc.

Really there with you on Debian bookworm!

Less with Flatpak. It is, IMHO, the wrong solution to a real problem; I install n flatpaks and suddenly I have n+1 openssl, libpng, etc. library versions to worry about, and unknown capabilities and policies for responding to security issues in each of them. Give me Debian unattended-upgrades any day!