i'm on krita next b1b691b for windows i see all the brushes, but i can only switch to my custom brushes. good luck on the rewrite, i'm rooting for you!
This is fixed in the model-rewrite branch (https://invent.kde.org/graphics/krita/-/merge_requests/303) That still needs some fixes to make tagging work, but I'm closing this issue now because it's cluttering up my todo list :-)