The general opinion is that if >70kg you can not move it adequately with 2x3nm motors so you need bigger motors, which now puts you into position to find bigger but at the same time fast enough or go with servo, which to me seemed the better option.

Now i am looking back at your gantry design. What size are the profiles, why 4, why are not connected each other? Whats your idea there? At that size machine
2x 100x100x3 or 100x100x4 welded together with separation for ball screw or not /if back/ will be ideal. 100x100x3 weights ~9kg/m and the 4mm weights 12kg/m, So you co8uld still move that gantry by 2x3Nm nema 23