What I was referring to is called a Bind Mount, where host directories are exposed to the docker container. You may be fine if it’s an external hard drive. I use bind mounts because they’re easier to back up, but I acknowledge they are less safe.
You may be perfectly fine as you are now. My (and others) suggestions are for added security. As it stands, if there’s no target on your bind, the only bad traffic you’ll get are from bots trying to pick away at your domain and sub domains. Generally they’re not a problem. But being extra safe costs nothing but time.
A third of the way is still not all of the way. You’d still save a few hundred, especially if you didn’t buy the base LCD model. For many people, this makes sense.