I just got it working. I had to change something else in another section of the category settings to get the button activated. Might have something to do with the migration.
Thanks a lot for showing the different solutions. I think we will try the core trust level system with some adjustments for level 2 and add two groups to the category permissions. For new users we would use trust level 2, for already qualified users from nodebb we add the migrated access group. That should work I guess. Thanks again, I already like Discourse 