Use true tools and aluminium oxide with water.

I can lap 12" disks flat to a twentieth of a wavelength of light. That's 1/20th of 633 nanometers. ie: 32nm peak to valley across the whole aperture. That's 0.0012".

It can be done, just takes a lot of skill.

See Engis or Hyprez for more advice, or any company that operates a Lapmaster.

Incidentaly, to go lower than a twentieth of a wavelength is difficult since light starts diffracting at that level and acts more like a particle than a wave. ie it's blinking hard to measure to that accuracy.