Hey Alan,
I think this would make sense.
This would better be a “Spindle/VFD” category since what is usually called a spindle in context of cnc machines is an induction motor which runs on three phase electricity and its speed is controlled by varying frequency so typically needs a VFD to be used.
But like “Aftermarket Controllers” there also could be a category “Aftermarket Milling Motors”. This may or may not include spindles. Such a category would make sense because in contrast to using a trim router, which has a 65 or 69 mm housing diameter to be held in a hand, there exist many reasonable solutions if people prefer using single-phase universal motors, like Mafell FM-1000 PV-ER (with interface to control speed) or Suhner UAD 25-RF (Suhner Catalog).
In contrast to trim routers like Makita RT0701C, such milling motors are allowed and made for permanent use in a cnc and they are delivered after run-in by manufacturer.
But at the moment it is clear that you can not use such motors with Onefinity since the Onefinity CNC lacks a 43 mm mount for wide-spread 43 mm milling motors 