I think you explained this to me months ago in the comments, but an update seems to have reset my changes and now I can't find the relevant comments (why doesn't Steam let you search the workshop comments section?), but how do you slow down or disable the passive decay of proficiency progress?

FWIW the default decay rate seems way, way too high. If you're trying to to learn through practice, you just have to grind it all out in one go in a way that feels really unnatural - otherwise you will lose all your progress by the time you get the next opportunity to practice.