On steel surfaces I routinely sanded long beams to about 0.02 mm flatness/straightness to mount linear guides in 25 / 35 mm sizes.
A 50 mm wide steel flat, sanded with a *wide* (Festool 105) powerful belt sander and 40 grit paper will easily remove material in a controlled manner, to about 0.01 mm control in depth.
The edges of the 50 mm flat get rounded, but the center part is flattish to better than linear guide specs of about 0.03 mm.
No scraping needed, nor shimming.
Using a ground machine-tool builders sharp straightedge, and a backlight, it is easy to see low/high spots on the surface.
Then mark the steel with a sharpie, one line for small removal, more parallel lines for more removal.
Then sand until the marks are faint, check, adjust.
It is fast, and very accurate, you get excellent control.
It is also noisy, dusty, unpleasant, hard work.
The wide belt keeps the sander from digging in.
If gross material removal is needed, a Hilti/similar small angle grinder and 45 grit flap wheel will remove metal fast, in a controlled manner.
Then belt sand to finish.
The grinder and flap wheel can cause tilts, so use judgement.