I have no experience of cutting at 21,000 rpm, I have dabbled with tiny tooling at 6,000 rpm. I will be curious to see if you succeed.

Soft aluminium alloys have a bad habit of extruding when they should be cutting. I find that when I drop below 8mm tooling I have to be careful when cutting soft extrusion if I don't want to snap tooling and that's with a good flow of suds to carry the chips away. I think things get a bit bendy at 6mm and below, the tool lacks the rigidity to insist on the cut.

Only a suggestion, but this stuff takes a deep T6 temper Blackburns Metals Limited - Mould Plate - Alumec