The ground loop isn't the problem, | adafruit | Page 1

OK, So I do want to close the ground loop, and connect the ground from the "upper" return leg on the left to the buck on that side. I wasn't sure, having done plenty of audio engineering in my time, and having learned about ground loops the hard way in that arena.
That said, the bucks are powered by a completely separate 20V system, using 14 gauge wire to run the 20V power to the rightmost buck. The left buck is very close to the 135W power supply, so I'm not worried about getting power to it.
That means the maximum distance I have to send 5V power in the neopixel strips themselves is 1/4 the length of the loop, and I've already tested that I can do that by driving a pair of 5M strips end to end from a buck powering them at one end. Even with all pixels on at maximum draw (20 ma in my case), there's no visible degradation of the colors.
I suspect that hard limiting the power draw of each neopixel will do a great deal to help with overall power delivery.

#The ground loop isn't the problem,