These are very edge-case, and I only mention it because you had a lot of helpful details that eliminated stuff, but could the gap between the extruder drive and the hotend opening be too far apart? I’ve had a problem kind of similar where it would jam up the filament there. Otherwise, the other outlandish thing I could think of is maybe the inside of the hotend is pitted and damaged, but that is not as likely.
I hope this helps at all, or maybe gives you some kind of idea. Good luck!
I tend to think left 4 dead’s Boomer zombie.
Slightly unrelated but when people say Zoomer, I often think about the hover bike from the first Jak and Daxter. XD