Maybe try something like this?
Basically: go into Plex Web, filter your music down to Punk (or whatever you want), and save that as a Smart Playlist. Teknojnky and I both suggested this earlier in the thread, as we’re both using Smart Playlists pretty extensively for the sort of stuff you’re after. It’s separate and entirely different from Mix Builder; it sounds like you might be conflating the two features?
If you have any trouble, feel free to hit me up to lend a hand, but I think this solution might resolve 99% of your current genre-related Plexamp issues, at least until the “genre shuffle” stuff is eventually available (hopefully soon).
