I was about to write something like:
https://dev.gnupg.org/source/gpg4win/browse/master/src/gpg-enable-keyboxd.bat
https://dev.gnupg.org/source/gpg4win/browse/master/src/gpg-disable-keyboxd.bat

For Linux, too. This made me realize that what I really wanted was for keyboxd to do this as an option / command.
I think this will be important if we do not want only new users to use keyboxd.

So on first start with an empty DB keyboxd should look for the pubring and import that.

I would add that as a subtask with priority normal. And if there is such functionality it should be available over assuan of course.
Second subtask with priorty low would then be to export back to pubring.kbx. So that would be for "disable-keyboxd" or if you switch versions.

If both these commands are available then I could simply have a script that calls first the import, then the export command over gpg-connect-agent to sync. The sync is not so important but would be nice for people that use both versions, maybe on different systems. But the import is IMO not only important but necessary.