@cass Hi. Welcome to the forum and thank you for your detailed request. You’ll be happy to know that this has been in the works for some time, and it’s getting there.
Please check the following thread to try an experimental version of Pencil2D with the MyPaint library implementation included.
This one fashions customizable brushes per drawing tool, and the brushes can be imported (albeit rudimentarily) or migrated to other open source software like MyPaint, Krita & The Gimp If I recall correctly.
If you have some time, please give it a try and do let the lead dev of this effort know about your experience. Thanks!