Greetings and CNC build soon to start, advice needed.

[JAZZCNC]

Hello Nick and Welcome,

Ok straight to the questions.

1610 is too small diameter at this length. You will get screw Whip at higher feeds. Better choice would be 2010 or better still 2020.
Reason why 2020 is better is because you can use ratio on the screw so it lowers the screw speed but still gives same speed as 2010. The ratio also gives another two advantages.? It increases motor torque so can use smaller motors. Lowers resonance to the motor/drives giving smoother performance.

This brings me nicely to your Motor choice. Nema 34 motors are not the correct choice for machine this size and type. They require much higher voltage to get decent RPM which you'll need for router. This means larger more expensive drivers and PSU.
3 or 4Nm Nema 23 are more than strong enough for router this size and if used with ratio like suggested then give excess power.

The Design I can't comment on because not really enough detail to see correctly. However I wouldnt use round type rails as they tend to wear quickly, lot less accurate and need lot more attention compared to square type . Which require very little attention and are super accurate/stiff.

Would also forget any idea of cutting Mild Steel with that design. Steel needs much stronger machine that what you have there.