Glad to hear. I used to have a Pi I used for headless server things but it got fried though I presume using a low quality PSU not meant for 24/7 use.
Was the GPU driver for Pi always closed source until now? Reverse engineering the driver may lead to some answers but that is a very time intensive task. In any case if the open source driver becomes unfriendly through dithering or other issues it should be easier to pinpoint what the culprit is. Also are you able to see any differences between closed source and open source driver? Maybe a camera with a macro lens would help?
Also I'm still using my Macbook Pro just fine still (the one with the M370X) It's become the best screen I've used in a long time, in fact I get weird neck pain from using it for 16 hours per day sometimes 😃 I remember you saying that you got eyestrain from AMD cards on boot but I wonder since the Macbook does not implement a legacy BIOS in any form (100% EFI) if that would cause problems for you. Also the Macbook has switchable graphics through a multiplexer chip hooked up between the AMD card, Intel card and the display. I'm working on reverse engineering it for switchable graphics under other OS.