Steel frame cnc router design/build

[JAZZCNC]

Z motor - probably didn't think it through quite correctly...with that arrangement, the y axis front plate is considerable shorter than it would need to be to have the leadscrew & motor attached.
The rail lengths are pretty much spot on compared to the screw length - maybe i've got the Z axis linear bearings spaced further apart than needed? (170mm outer face to outer face along the axis). I'll look at swapping the leadscrew & motor to the Y front plate.

With 350mm Thread length and only 400mm rails then you have loads of excess thread. You only need enough thread for required travel + ballnut length +10mm or so spare.
RM1605 nuts are approx 50mm in length so for 180mm travel then 250mm thread length is plenty.!

Base frame - do you mean raise the lower shelf/surface, or reduce the overall height?

Yes raise it and widen bracing to match. You don't want to rely on bed frame to form main frame strength, obviously it will add strength but better is main frame is strong enough without.!