Ok, I just ran the e-steps calibration and oh my god, I’m 99% sure that was it.
My E-value was like 345, and the first time I ran the test, it extruded 140mm instead of 100.
After a few iterations, the dialed-in value was 233.33, so yeah, extruding 40% more material than desired could generously be called “overextrusion”. 
BTW, I think what happened is that I ran into all kinds of problems with the original hot end, which I now think was fairly well blocked. I had run the extruder calibration with that, not knowing it was blocked, so yes, the crazy-high E-step value was probably reasonably correct…for a mostly-clogged nozzle. When I replaced it with a brand new hot end, I was suddenly pushing way too much filament.
So the reminder to self and other newbies is - when you swap in a new hot end, you should at least re-run the extruder calibration and re-level the bed. I wonder if any of the pros have other suggestions.
Running bed calibration now, and working on slicer flow calibration, but I think we’re over the hump for the most part. Hearts for everyone in the thread!
Here’s the slicer flow calibration cube. I thought we all deserved an “After” picture. Smooth as silk.
