I often add my own tags to an album. Or series of albums. I do this with the Picard scripting.
$setmulti(genre,MyTag; %genre%)
It is a dumb simple script I turn on when I need it. Then just overtype “MyTag” with what I want to add to the tracks. It adds my custom tag to the other tags.
OR if you want to just set all track on an album to have the same four tags of “Alternative Country, Americana, Country, Rock” you can do this in the GUI. Use cursor to select the ALBUM now you can manually update the tags for all tracks of that album in one go. Then hit Save