How would you feel about just making a smaller Y and then moving the material through and doing multiple cuts?
If you imagine a 71" piece of conduit, and the router was biting or kicking back in the X direction, the stepper motors for the X could be 35" away from the bit. I think it would have a hard time fighting the aluminum.
With good CAM, the loads are very small, but it will occasionally bite, and when it does, if it moves too much, it will have way higher load, and you’ll get a bunch of chatter.
I love the idea of building an MPCNC with a first team. I think it is not only going to expand the kind of things you can build, and give the team experience with CAM/CNC. It will also be a good example of a bunch of interesting robotic solutions.
I think you’d be very happy with about 12"x20"x2.5" cutting area and a dw660. I think it would be very forgiving for all the learning curves it will have to handle. You would just cut 1/3 of the 60" at a time (and then flip it to do another side). You can also build your own connector pieces out of larger plates. Maybe out of ACM.
I am not an expert of metal, so I don’t know if that kind of aluminum is hard or easy to CNC.