That's all pretty much true, but it is mainly describing how it can work
after you have all this thought well-through, set it up, and have entered all genres and sub-genres for the files in your library.
The main objective of this request is making it easily accessible, easier to manage, and more inviting to use, also for less-savvy users.
One obstruction that makes sub-genres not an easy and welcoming feature at this moment, is already at the stage of entering them.
You might have a list of sub-genres you want to use. (you will definitely need one if you want the slightest chance of some consistency).
Currently for every track or album you would want to enter a sub-genre for, you would need to consult such a list. That is not a fun thing to do every time, and one of the reasons I stopped (paused) pursuing this, and probably one of the most important reasons for many others not to pursue this, even if they would really like to.
The second obstruction is that MusicBee has no understanding of sub-genres. When using a custom tag for it, MB will allow any sub-genre to be entered in relation to any genre without any restriction, warning, or any 'awareness' of relationships.
There would be no indication at all that e.g. it would be strange to enter 'Late baroque' as sub-genre for a Hip-hop track.
All you can do is either enter a sub-genre manually from 'your list', or choose from some entries from a pop-up list that might contain sub-genres that were entered earlier before. But there is no filter, restriction, comprehensive list, or suggestions for sub-genres that have a relation to, and falls under the concerning genre.
The wish/proposal for the above would be:
1.
To be able to insert (paste) a list of genres, sub-genres, and styles¹, or have it integrated in MB to start with.
¹ 'Styles' is probably more simple, because it would not depend on relations, since for example 'industrial' or 'avant-garde' may be used across many different genres.
2.
Having it easy to find, select and enter genre, sub-genre and style tags for a track or album.
For that, possibly some dedicated genre/sub-genre/style panel would be very helpful.
To give some rough idea, maybe something like this mock-up. (which I also posted in the other thread) :

In my imagination this would work something like this:
You have an album or a track that you want to populate with genre and possibly sub-genre and style.
1. You begin selecting a genre category. (that should be easy)
2. Then from the drop-down menu you
must select one of the genres that is presented there. Only relevant genres to the selected genre category are displayed.
3. Save and exit, or go to the
optional next step.
4. From the sub-genre menu, select one of the displayed (and already filtered) sub-genres of the genre selected in the step before.
5. Save and exit, or go to the
optional next step.
4. From the Form/style menu, select one (or more) item(s). The contents of that menu will not depend on earlier selections and has the same items every time.
5. Save and exit.
Since many users will want to keep using the current implementation for genre, it would probably be best to have it optional to activate such an 'advanced sub-genre/style mode'.
At activating it you could then choose the mapping for sub-genre (most likely a custom tag) and 'style' (the 'keywords' tag would be a likely candidate, or else a custom tag)