I have never attempted this but to me it sounds like it might lead to weird issues if nfs is slow for some reason and it will definitely make your nextcloud instance slower. You can easily just try it out though. Copy the docker compose project, start another mariadb instance with its data directory mounted from the nas. That way you can figure it out yourself.

Personally I would just back up the database to the nas regularly. Yes if your laptop fails this requires copying data. You already need to install an os on the new laptop and pull the git repo so having one more step of copying the data should be easily doable in my opinion. Especially since you hopefully don’t plan on having your laptop break all the time.

the thing can run on your premises completely

lighthouses that do the initial intros

Sounds like these lighthouses must be provided by somebody else which makes it not entirely on my premises anymore.