I started with the yunohost install as well but decided to migrate to the officially supported docker install when I had issues with upgrades or had to wait for the ynh package to catch up. Although I don’t want to take your money and bear the responsability of handling the migration, I could give you some pointers.

• Use the official backup and restore documentation as an inspiration
• Use the -W option with postgres commands so that you get prompted for the password
• Don’t use pg_dumpall but pg_dump instead and select the lemmy db
• First migrate to the same version on docker
• Make sure everything is working fine
• Then proceed to upgrading to latest (depending on which version you come from maybe it would be safer to use intermediate releases)

That’s from the top of my head but I had to try a few times before getting it right.