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!

What retraction settings do you have? I’m wondering if maybe that’s contributing to your jamming.

I’d personally try a sample coil from somewhere to rule out potential filament age/moisture/contamination.

SOLUTION: runs contrary to common knowledge, but what ultimately helped was:

• INCREASING print speed. The usual recommendation is the slower the better, but at that point, there’s no pressure control over the filament
• ENABLING retraction. Again, usual rec is to disable it for flexibles but turns out it alleviates too much backpressure which leads to filament buckling in the extruder

Also printing hotter than mfg recommends: I printed at 260 (10C over recommended maximum).

Thanks to u/Over_Pizza_2578 for pointing me to the right - opposite to everything else - direction.