retrofitting a Proxon for coin die milling

As I'm planning to do live coin minting at some events I attend, I will retrofit a proxon milling thing to make my own coin dies.
Model is this one:
forum.drucktipps3d.de/file-download/12960/
I'm in contact with a nice austrian guy, who has done that before LinuxCNC exists (uses some old DOS software via par port). So the mechanical part is quite proven.
Plan is as follows:
- replace all threads with ballscrews
- a mesa 7i96s (have that from a dead project)
- stepper motor and microstepper driver (from yet another dead project)
- maybe place a 1:6 gear between stepper and ballscrew (this is actually trick from the austrian guy) for higher detail
My main question is about the milling spindle. As I will mill little amounts of material, I was aiming for some small spindle (partially for lower weight to have faster and more precise movements). On the other hand it should have quite high rpm and very good concentricity.

So far I was thinking a AMB 530 FME. At least I couldn't find a lighter one that would serve the purpose.

Please let me know if my plan might generate a working machine or if I have a major f-put in my plan.

ballscrews ar primary because the original threads in brass nuts have quite some deadway. But yes, it is all metal and the swallow tail rails are even adjustable. Plus it is the same model the other guy was using for years.
Only difference so far: He used a setup, where the motor was separated from the machine with a belt transmission and the z-axe was only moving the milling bit.

Also: I thought I might use one of my older PC standing around, but the latency test told me other wise (150ms and above).
Next try raspi4...

No not that old. But maybe I lack some general understanding.
1. my raspi4 gets an base thread may jitter of 126 ms (126000 ns), which is supposed to be not usable for linuxCNC ( so why is there an image for that?)
2. or is the mesacard exactly there to address that problem with it's FPGA?

Sorry for the somewhat below rookie-level questions