Yes, "bedslinging" is the proper way to build a milling machine for metals. Gantry designs exist for CNC woodworking, but are uncommon for metal.

Do not use 2 leadscrews per axis. You cannot time them well enough to prevent binding over long moves. If you are worried about moving the mass with the motors and leadscrews, buy bigger ones. The best way to drive the Y axis is with another stepper and leadscrew.

You can drive ballscrews with belts if you need to offset the motors. But your belts need to be as short as possible and as stiff as possible. Otherwise you will lose steps and make junk rather than functional parts. So no long light duty toothed belts like 3D printers use.

And when thinking about motion despite what your eyes tell you, it's always the tool that moves and not the part.