How old is the filament? If you've had it a long time perhaps its taken on too much water (once things get to a certain point it can irreversibly make the filament print poorly, even if you try to dry it)

I'm not super knowledgeable myself, but I hope someone here is able to provide useful guidance

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

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.

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.

Are you using a bi-metallic headbreak? I've heard these can make it very difficult to print with TPU (at least with my Volcano style extruder on the Sidewinder X2), though I don't have any experience with it myself.

So, one thing to consider is whether you could have remnants of a previous filament getting stuck in your hotend and carbonizing and causing partial clogs. That depends a bit on what you were running just before running this filament. If this is the issue, a cleaning filament can help. Another possibility is that your nozzle isn't tight against the heatbreak, in which case plastic can accumulate there and cause issues. The only way to fix that is to disassemble the hotend and put it back together correctly--this usually involves tightening the nozzle when it's hot, but check the details for your specific printer.

But yeah, it could also just be bad filament. That's probably the easiest thing to fix, anyway.