Hey Nathan, hey Echd,
When I imagine your situation, Nathan, i.e. you have another CNC and the Onefinity is already there ready to be assembled, at same time a spindle is too expensive and a dedicated milling motor probably too and also the latter cannot (yet) be mounted on the Onefinity, I ask myself if @Echd’s advice is not so bad.
I’m usually the first person who warns that the Makita hand trim router is not the right thing for a CNC machine, and that it’s easy to overload it in a CNC, and unfortunately it doesn’t trip in that case (like a VFD would) but gets too hot and can burn, but regarding what Echd says, if you have experience with a hand trim router (in hand-held, short-duration use), then you pretty much know under what conditions it will work without overload (and not burn out), so if you make sure it stays within those conditions you can use it to test the CNC machine for the time being (until you have money for a spindle). Possibly better than having the machine around doing nothing. You could get familiar with the machine this way.
Of course it depends on what projects you intend to mill with it. But it seems obvious to me that many Onefinity CNC owners only do things that don’t burn out the Makita hand trim router. In any case, with a hand trim router mounted in the CNC, I would always stand besides the machine when it runs (of course with a big (e.g. paddle-style) emergency stop disconnecting the entire installation (=not the red button on Onefinity Controller case) and a fire extinguisher ready) and avoid using bits larger than 1/4". When a Makita burned out, it was often when users used 1¼" surfacing bits.
As explained here, the Makita has no torque at high speeds (and wood is milled at high speeds) and with this type of motor, the speed decreases with the load, and when slowed down the current increases, and with it, the heat, while the fan then turns too slowly. So you know what to avoid: Too high load. Also I would not use bits larger than 1/2" with the Makita (especially avoid large diameter surfacing bits as it was always during their use that the router burned up). Makita does not offer bits larger that 1/2" for the RT070x.
What is bit sad and often leads to problems is that the proprietary Makita collet does not hold the bit as safely as on an ER collet.
Further reading
The Makita router ate the big log – Post #1 by Bear
Makita Router Overload – Post #1 by RexH
Fire Safety in workshop – Post #9 by Aiph5u
Largest clearance bit the Makita router can use – Post #2 by Aiph5u
Makita Bearing went Bad – #4 by ConvenientWoodwork
Makita burned up – Post #1 by WikiSnapper