Hello. I recently did a mod where I put a G403 PCB into the WMO.
Microsoft WMO G403 Hero mod : MouseReview (reddit.com)
Here you can see it. It's fairly simple if you know how to solder.
WOW man, this looks great. Once I did a g102 mod in my WMO. But, unfortunately, it only lasted 2 days. Its scroll wheel was so delicately soldered that it was broken. It seems like you didn't change its left, middle, and right buttons' PCB. If you have more pics of the pcb - how you did it and share it that would be great.