Hey Bill, hey Nick, hey all,
The spindles I refered to above do have maximum torque at 1,500 rsp. 3,000 rpm, both the HPFD and HFPM series are designed to mill steel (see the torque/speed diagram in the datasheets).
However a spindle, which is an induction motor, can never compare to the router, which is a so-called universal motor (a commutated series-wound motor). Only a spindle delivers constant high torque over its entire rated speed range (CT mode), while with the router, where the speed is dependent on the load, you only have an extremely narrow combination between usable torque and speed. Also a spindle is never slowed down by any load, unlike the router. If you want to know about the fundamental differences between the two, see here
You know I’ve often questioned how an experienced machinist like you, of all people, still works with this hand trim router instead of a spindle. However, knowing the kind of work you do, that could be why you’re still happy with the router: first, perhaps that narrow torque/speed range happens to be one you need, and second, you’re probably very far from demanding much power from the motor 
, and of course thirdly because the spindles usually shown here are not rated for speeds below 6,000 rpm, as they target people who want to mill wood. However the hand trim router does not offer you low speeds either.
But I totally agree that if you put one of the spindles that are able to deliver high torque at very low speeds, and which are designed to mill steel, on the Onefinity, the Onefinity is the limiting factor then, at least if you expect steel to be milled at this speed and at this MRR, as also impressively shown here. It is not a machine designed for it.
Do you remember when we discussed another interesting machine here and here, we described the problem on the Onefinity with the router slid far downwards into the mount, the problem with the Z assembly in general, but also that the other machine refered to, which is sold as being a versatile machine perfectly able to mill steel, has plain bearings instead of linear ball bearings which would be more resistant against the high forces and vibrations when milling steel (by the way that machine had some other advantages)
What a nice machine. Why did you sell it? Your Onefinity Machinist isn’t really able to replace it (in no case in matters of milling steel)?
In this mill, as they describe here, you have a type of motor that is far superior to the commutated (brushed) universal motor like your hand trim router. A high torque from 100 – 2500 rpm, and totally silent, that’s ideal.
However the power they show here is sufficient for a mill. For a CNC, where you have much more speed of operation, it would have to be much stronger.
Note: If one should not only be interested in what spindle manufacturers offer to consumers and hobbyists, like Mechatron does, it is worth a look at this manufacturers list.