Also about the mistagged/untagged breasts tag, what's your plans on it. Should users use their manpower gardening that seemingly futile effort?
It's up to you. Every common tag has thousands to tens of thousands of untagged posts. AI tags were added exactly so these posts could be found. Breasts is not unique in this regard, it's literally every large tag on the site that has thousands of untagged posts. Which ones you want to spend your time on is up to you.
If you try to garden any large tag like breasts, what you'll probably find is most of them aren't worth the effort. Pick any common tag like smile or blush or looking at viewer or open mouth and you can easily find tens of thousands of posts where you could add the tag. But spend a few minutes on this and you'll quickly realize there's no point if you never search for these tags to begin with. You're better off picking tags you actually do search for. If you want to garden breasts, it's a lot easier to start with more specific tags like cleavage or nipples or huge breasts or sideboob, that a person might actually want to search for, rather than breasts itself, which is so broad and commonplace that it applies to half the site.
Or you could just search for ai:breasts directly and not rely on humans to tag it. Part of the point of AI tags is to allow people to search for things even if a human fails to tag it. So from a certain point of view, it doesn't even matter if breasts is untagged if you never search for it and if you're still able to find it with ai:breasts even if other people fail to tag it.