Maybe a rebuild will fix whatever is causing this. Because when you experience:

1. Stuck filament
2. Underextrusion

That does sound like a strictly a failure in the filament movement system. And I can only think of

1. Grinding extruder, which can also be a symptom of the following
2. Unstable temperature
3. Clogged nozzle

And since clearing the nozzle didn’t work, and you don’t find signs of grinding, nor observe temperature swings, then I’m out of ideas.

Do you happen to have spare nozzles? Maybe try swapping the nozzle and see if it was still clogged? I’ve had nozzles that I just couldn’t clear.