Not read the whole thread but your initial design is incredibly similar to how I just built my router and I can say from experience don't fit the rails on the gantry that way! You lose too much travel.

I lost almost 28cm of Y travel because of how I built the Z axis at some point I will rebuild it so I only lose 15cm but luckily I built it oversized so I still 1.2m*0.7m which is enough to do what I want for now.