Yes, well, lesson learned there, my apologies. I thought people might be happier with a separate topic since I was sort of changing to talking about the power supply switch. On the plus side, I've never heard anyone call a Macbook a Fruitbox, and that made me laugh.
If you have any thoughts about other tests I could do or things to look at to eliminate sources of flicker, I'm all ears.
Here's what I'm trying:
Can try lowering current via dot correction
Can try altering latching delay in software
Can try Adafruit power supply
Can try big cap directly across supply to chain
Can try Arduino sketch without USB connection to rule out interference from USB power/gnd/data/serial