What is the minimum backup for a home server

The data volumes and the compose files, nothing else. Containers are rebuildable; the database and the uploads are not. A nightly restic snapshot to a second machine plus a weekly copy offsite covers the realistic failure modes: disk death, deletion, and theft.

It commits you to a format that is tedious to migrate away from later. The first weeks also look worse than doing nothing, which is when most people abandon it.