Okay, here's a mid-project update.

I printed Benchy. Well, the bottom of Benchy, since everything above the hull detached and looked more like a ball of wool than a boat...

So, I spent most of the early afternoon taking apart the entire thing again. I ensured the nozzle was clean (it was), re-leveled the crappy printing arm (these are supported on only one side) to discover the whole thing was leaning by about 8 degrees, and examined the tube that brings the filament to the hotend. The tube seemed rough inside, so I replaced it. The feeder gear (sorry, I still don't know what its called lol) was coated with compressed filament powder, and I noticed that after that gear all filament was slightly rough.

At that discovery, I figured the feeder was putting too much pressure on the filament, which was deforming it, creating sharp snags. Those snags likely caught whenever the hotend moved, causing the filament to stop extruding.

After lowering the feeder tension (it's an upgraded one I printed a few months ago), I started Benchy again.

So far it looks back to normal. I'll add the pics to this post once it finishes. Might be worth adding what I found to an under-extrusion troubleshooting guide?