Mine's a 2020, but I don't think they changed anything from 2019. Same setup worked fine on the 2015 version.
I'm using a 64GB thumb drive formatted FAT32, all 256 KBPS MP3 files, about 8000 songs.
File structure is Artist/Album/Song.
I don't use playlists. Everything is working great. I have album art, can play by artist, album, etc. Generally keep it on random, but I don't use the bike stereo too often.
This is the same setup I use in all of our vehicles, except I upped the size in the cages to 256GB when they were on sale recently. 256 is overkill, but 64 is not quite big enough to hold everything.
The ID3 tags is what makes everything work in these systems. When I first set this up I spent a lot of time getting the tags right and I make sure that everything added has the tags right. With proper tags the file structure really doesn't matter, so you can set it up however is most convenient. MP3Tag is a great, free program to fix tag and add album art.
2020 Road Glide Special - Billiard Red - 114 CI Stage 2