I can only move messages to a root namespace folder when specifying inbox=yes. That is strange because it is not even the inbox. I don't even get why it is not by default possible, maybe bug there?
You have made the mistake of overloading the use of a parameter inbox=yes (or not creating a parameter to tell folder can be used to move messages to), there is indeed only one folder that gets incoming messages delivered to, and that is the inbox=yes. You should have created a 2nd parameter specifying if a folder should be allowed to store mail.
(I have been running this dovecot server for 20 years or so, and I am really surprised about still running into issues on such basic things as trying to put folders onto different storage. Not to mention the lack of help you can get here. Is there some roadmap, features implementation overview available? Can you make public issues on github?)