I have a wide variety of brushes picked up from Humble Bundles, individual artists, and courses over the years. Frankly, I don't know how to properly use the half of them, but part of me at least wants to ensure that they're available to me if, say, I come upon a point where I need a brush for cracked skin. I know there's the search function, but that only works with the current library.
Organizing brushes is a very personal thing because we all think differently about remembering and organizing....so one person's method may or may not work for you...you will need to determine for yourself.
But seeing others may trigger ideas for your own system.
I've seen 2 very successful methods:
1 is to make custom palettes, each organized by projects, or brush types, or favorites, or any ideas you have. They can be turned on/off easily and can contain stuff other than brushes.
2 another is to use brush libraries and categories. Libraries can be by brush types or any method you choose; same for categories within libraries. Painter has a convenient brush selector panel already available...but it only contains brushes. That is Corel's suggested method.
I use the library/category method and libraries by 2 basic types...all in more or less chronological order. For each Painter version such as Painter 2018, 2019, 2020, etc. it has its own default library. What ever I add to that version I add a new library 2018+, 2019+, 2020+ etc. So as to keep the default "pure" or uncontaminated.
I have categories in the "+" libraries that make sense to me. I have a few other special libraries for collecting certain brush types: watercolors, sumi, thick paint, etc. As time goes by and new versions come long I simply import previous years of libraries into the newest version of Painter, but often do not import the oldest as old brushes are replaced with newer features and no need for older brushes...but they are there saved as backup if ever needed.
So your painting style, organizational methods, etc will dominate how you organize...as well as how well you can remember your brushes for what they do and their names. And often a brush by a given name can do so much more if it were thought of as being in a different category. And in P21 we can mix brush types more easily on same layer or change layer type to accommodate varieties/mixtures.
Some people make an example brush stroke image for each category for ref.
I use a search engine for my PC called "Everything" and make some pre-searched .pdf's of brushes that can be searched internally.
So, for example, if you have an idea you need a cloud brush but can't remember where they all are; simply do a HDD (all of them) search for cloud .xml and they can all be found and which path (library and category) and name of any cloud brushes. (.xml finds the brush. .jpg finds the dab, there is also .nib and .stk internal for Painter).
Search also works well for papers, textures, libraries, categories, etc.
Windows search in file explorer also works well, especially if indexing is setup to make searching much faster. Go to 'windows search settings'.