I agree. AF needs to have better brush management capabilities. It was something Photoshop users had to wait a very long time on. I hope AF doesn't let us wait as long as PS did with their costumers!
I suggest making it possible to create sub-categories within categories. A 'sort by ...' button in the hamburger menu would also be helpful (for example 'sort alphabetically'). Another thing that would make brushes more manageable is to let users choose how they want the brushes to appear in the brush panel (by stroke only, by name only, by brush tip only, ... or a combination of these things). Also having multiple brushes sit aside each other when resizing the brush panel would be helpful for those with a very large brush collection. Maybe instead of selecting the collections in a list, icons could be placed on the left of the brush panel, which could be scrolled through, and with dividers, making it quicker to switch back and forth between different categories. Or maybe only the categories favorited by the user could appear here. Beneath is a quick mockup of what I mean.