Not something so complex that it requires a docker

Docker is the thing that sandboxes your services from the host OS. I’d rather use Podman because of the true non-root mode, but Docker is still based. Plus, you can use Docker Swarm if you don’t want to switch to Kubernetes (though you don’t have easy storage integration for persistence).

Buy a VPS, set up Nextcloud and try it that way.

Alright then. I’d recommend Nextcloud.

Obsidian has a built-in sync feature. Use it.