Brushes and Overlays offered by Joel Grimes

I am not familiar with them, but they look interesting for just $19

Brush sets that I have boaught and been pleased with include various Aaron Blaise brush and texture sets.  They are only a few dollars, and amazingly good.  Especially the hair and fur brushes, but also his cloud brushes.  Aaron has video guides on how to use them which will lift your skills. 

https://creatureartteacher.com/product-category/tutorials-lessons/brush-texture-sets/

I paid a bit more for a bunch of Photoshop assets from https://www.nucly.com/ but I don't use thrm as much as the Aaron Blaise presets.  

A bigger investment were the Boris FX Optics.  It contains a zillion effects but it takes a while to get to grips with them all. Much the same with ParticalShop.  it has a nicer UI than Optics, but here again, I don't use it a whole lot, and I find I have to reguarly practice to keep my hand in — so to speak.

OK, I have had a better look through the Joel Grimes collection, and decided I can already do at least as well with what I already have.  Some of the NG effects can be achieved with presets from Kyle Webster's Megapack.  The problem with the Megapack is that is contains something like 700 presets, so it is absolutely worth going though it and copying the most useful presets to a new brush group.

I don't know if this has helped, but we don't know exactly what sort of looks you are interested in.  I have built up a reasonably big collection of Photoshop assets over the years, and I do use them, so if yo think a set will suite your illustration style, I would say go for it.  But definitely look around for alternatives first, and especially the Aaron Blaise assets.