Docker is fantastic but there is definitely a learning curve. I use docker compose for everything, which makes it easier (especially blowing away and recreating a container).

Instead of trying to work out where stuff lives, I create bind mounts. Basically, map a directory on your system to a directory within the container. Most self-hosted services these days will provide docker-compose.yml example files with bind mounts specified, and docker now ships with compose baked in.

I create one directory to hold everything, then one directory to hold a service, then put the docker-compose.yml file in there. Then I can back up just that one directory and everything important is backed up (I do things differently for lemmy because I want live backups, but all my personal stuff is done like that).