maciuc I don't think there's any solution to your situation that doesn't involve removing tags to achieve a result.
You could make all your users start new discussions in a special restricted tag, and then move it to another tag when it should become public. You could also use the extension API to automatically add a specific secondary tag to all new discussions so users don't have to select it.
If you want to keep the additional tags for SEO/structure, you could remove permissions from those tags and instead apply permissions to secondary tags. You could then even hide those secondary tags with CSS or something.
Basically, with Flarum's native permission system, as soon as you add a tag with restricted access, it will take precedence over everything else. The restricted tag can't stay on the discussion if you want to make it visible to more people.