@fbartels that is correct, admins can login to all apps always. This was the case from the get-go and now it is a bit hard to change this behavior since setups depend on this. But maybe worth reevaluating.
@rosano 2GB is the minimum for a single Discourse app, if you have other apps running as well, it's probably not enough - so yes, more RAM = better. I haven't tried a migration, but installing and setting it up worked fine.