Threaded rods or allthreads as they also are called are commonly available in hardware stores.
I buy them in 1 meter lengths here in Sweden, galvanized and stainless and various threads (I mostly use M8)
They are for example used on many 3d printers for Z axis…
The quality of them (precision) is not always top notch - so I run either a few nuts along the thread or even better a cutting die to smooth it out.
As many are not perfectly straight some sort of floating arrangement could be handy.
The slowest axis on MPCNC is Z (T8 Leadscrew) …your machine will be slower on X and Y compared to belts and that is direct drive from motor.
M8 is (1.25mm) * motor RPM = top speed in mm
But I just thought…using them as racks…you will not turn the threaded rod to get movement?
is this something that you want?