I've got one word for ya, Nemezis: Coolant.
And a second: Enclosure.
Let me tell you a story.
A couple of years back, I joined a hackspace. Mainly because they had a laser cutter that I could use to make custom acrylic chassis disks on, rather than buy the ones from TCSS. They also had a 3040 desktop CNC engraver, which they found out could cut aluminium, because the bed was aluminium, and if you got the z axis depth wrong, the bits would go right through it.
So, I heard that, and I figured "well, now not only can I make my own chassis disks, but soon, I can make my own MHS-like parts! I'll be free from TCSS!"
I spent a few weeks playing around with this not great CNC machine and fusion 360. Learned the basics, feeds and speeds, flutes, etc. And if I remember right, my numbers were very close to yours. But, the spindle on the machine couldn't go past 7500RPM. The 4mm, 2 flute cutter was using just wasn't cutting well at that speed, and the machine really wasn't designed for cutting metal, so larger bits with slower RPMs would make the whole thing vibrate and cut worse.
And then I tried applying a lot of 3-in-1 Machine oil on my work, and the quality of the machining went through the roof. so, I bought a few cans of 3-in-1, and started trying to mill it into a copy of a ribbed-grooved MHS extension. it took about two hours, was ridiculously noisy, I went through three whole cans of oil, and in the end the piece was a fail because I'd got the mirroring wrong.
and then came time to clean this pool of machine oil and chips off the workbench. and the CNC. and the machine room walls. and ceiling. and the toilet next door.
I didn't do a great job of it. I thought I did, but what I failed to account for was that there was still plenty of oil that had accumulated in the nooks and crannies of the CNC, which seeped out during the night.
Did I mention we had a fire inspection the following morning? Surprisingly, the firefighters take a very dim view of pools of flammable material under machines powered by electricity.
So, that was the end of that, and as we've yet to figure out a proper, DIY recirculating enclosure that won't get in the way of using the CNC for other materials (wood, plastics, etc) I haven't gone back to it. Besides, I'm having more fun with 3D printing.
But if there's to take from that pointless tale, it's 1: cooling will improve the quality of your cuts in metal, 2: it'll make a huge mess, so try and keep it as enclosed as possible.
also 3, never apologise for using the superior system of measurements.