I like the idea here, allot.
My $0.02
In my figuring the gantry has no way to stay square especially along that length. I’d think if you were anywhere other than the middle and applied a lateral load the two vertical guide rods would go out of square with the horizontal and possibly not even stay parallel to each other.
I think you need a piece that fixes each end of the vertical much like the brackets you have for the router (something rigid with diagonals. In my estimation that’d keep the ends square and let the whole gantry move as a single cohesive unit.
But again this is just my thinking, you’re the one actually doing!