I can help you with part of the answer.
You can export a playlist for only channels with HDTV tags (for example) with
http://localhost:9981/playlist/tag/HDTV
I'm not sure if you can join or unite them. I've tried
http://localhost:9981/playlist/tag/HDTV&UHD
and HDTV&&UHD and even HDTV+UHD but they didn't work. Not sure where the documentation for this is either.
If you export one after the other, VLC (If that's your player), will then append the lists together though.