Think it’s all sorted out. From what it looks like, it was the extruder spring being way too stiff. The current spring feels like a stock Ender 3 extruder spring, or possibly a hair stiffer because it didn’t need spacers and the stock one often does.
The upgrades I ended up doing: Winsinn extruder (it’s red), Capricorn TL tube, and improved PTFE tube couplings. The Winsinn extruder takes a larger coupling, but thankfully it’s included. Calibrated the esteps (140.3 for mine, which is around average for the Winsinn), re- ran a temperature tower, and then started doing floor tiles. I now have three: one from yesterday and two more from this morning. So I think this problem is solved?
One caveat on the Winsinn extruder: there’s reports that some are shipping with the wrong idler bearing retainer screw. Mine shipped with the correct one- it has only a few millimeters of thread and the rest is smooth (I don’t have a part number, sorry). The wrong screw is threaded all the way and causes premature bearing wear. I’m mentioning it here because I don’t think any of the video reviews address it.