Got the parts on eBay for $140 shipped (when they had 10% coupon), installed today on my 2018 CX-9 GT, took ~3h and only because I was futzing around with looking up stuff while doing it. It was minimally difficult, but you can do some cosmetic damage if not careful - the forum and instructions were very helpful, though wish there was an actual install video.
Best firmware install video I found -
this one (from Mazda3Revolution forum).
Used these two videos for the install -
overall install and
screen and CMU removal (from this forum).
I mostly followed post #205: 1) prepped the wires with the foam tape per printed instructions, 2) disassembled rear vent/trim/glovebox/screen/CMU, 3) ran and zip-tied cable and 4) reassembled. I did not do step 9 (remove HVAC duct piece, not sure why needed).
I did use a trim removal kit, but I think it's very possible without it if careful. I didn't realize there was further disassembly after you pull the screen out to take the CMU (and biggest chance to gouge your dash), the hardest part for me was getting the CMU back in as the cables were getting in the way.
Quick tip on the USB hub - I took it out together with the trim piece around it (there are two tabs on each side you squeeze to do that), disconnected the cables, then pushed in the 4 locking tabs for the USB hub itself from underneath. Otherwise, I think I would have damaged the trim around the hub.
Used the tape measure method to run the cable from the front to the rear - pushed the tape measure through to the front (might take you a few tries), taped the cables fully and tightly to the tape measure, and pulled it back through (you'll need to pull fairly hard).
No issues so far, will be testing it on a long road trip tomorrow, pretty excited!