Hey Art,
I consider the entire Z assembly as the weakest point of all Onefinity machines and intend to replace it for this reason.
But to answer your question: The Z assembly differs strongly from other CNC machines by its unability to protrude the milling motor mount downwards beyond the end of the Z linear rails. See here to understand this. This means the absolute position of the bit end can only be influenced by 1. lowering the entire Z assembly in its one of four possible mounting positions, thereby loosing clearance under the gantry (or, with the same result, by increasing the thickness of the wasteboard to lift your workpiece to a higher position), and 2. sliding the cylindrical milling motor more downwards inside the milling motor mount, which increases the leverage forces exerted on it during milling which can lead to chatter and the accuracy of the milling result is reduced, i.e. the play at the end of the bit is increased.
This means if you mount the milling motor (be it the hand trim router, or a spindle) at a position near its lower end (which would usually be considered as recommended to keep lever short and chatter low), you have no chance to reach the wasteboard. This is because of the Onefinity Z assembly design, which is unable to let the milling motor mount protrude downwards beyond the lower end of the Z linear rails, as shown in the link above.
If you look at what I consider the best milling motor solution for hobbyists and semiprofessionals, their mounting position is exclusively at the lower end, at the height of the bearings. This is the best position to clamp a milling motor. With these milling motors, it is impossible to slide them downwards inside the milling motor mount. This, of course, makes them incompatible with the Onefinity style of millling motor mount. What you would need is something like shown here (see also here and here). It’s a feature requested since 2023-02-27.
To come back to your question: If you keep the Onefinity stock Z assembly, it’s up to you to find the balance between sliding the milling motor downwards to reach your workpiece, but increasing lever and the danger of chatter or inaccuracy of the milling results, or, fixing the milling motor near to its lower end to improve accuracy but are reducing effective usable Z travel.
But while you can slide the hand trim router downwards inside the milling motor mount to its very end (you are even forced to in order to reach your workpiece), spindles usually have a region marked where they can be clamped. You may damage them if you slide them too much downwards inside the milling motor mount.