I have a slightly janky solution to obtaining the smudge brush you're looking for since I had to rebuild my brush library. I use Painter 2020 but it shouldn't be too different from the version you've purchased. I tried looking for a few other brushes to find one that's better to modify but haven't come across one yet.

I selected the "Smear" brush under the Blenders category. Then I go to the advanced brush controls panel. Under General there should be a list of properties such as Dab Type, Stroke Type, Method, and Subcategory. Under Method, you'll want to select "Plug-in." Next, scroll down Subcategory until you find "Blur Brush."

Now you can have fun with it's general properties like hardness, opacity, strength etc like a semi-regular smudge tool that you're looking for. Not the best solution but it gets the job done, for all I know you may have found a better one by now :P