Hey Bill, hey Terje,
but the other day we discussed that the need to slide the router or a 65 mm spindle that much downwards in the mount, with the result that the distance from bit to mount becomes “5″ish” as you called it (=12.7-cm-ish 
), because otherwise the milling motor’s back would bump the Z stepper cage, is a strong disadvantage in matters of rigidity and chatterproofness.
But you are right, the opposite, I mean zero distance, would be a loss of Z travel. But that is due to the design of the Onefinity milling motor mount. It differs much from the usual spindle mounting plates (see links below) in that it does not protrude downward beyond the Z-rails end. Not at all. It leaves it up to the milling motor to protrude the Z assembly downward, and that is a severe disadvantage that costs Z travel if you want the collet to be close to the milling motor mount for matters of rigidity.
Usually gantry-type CNC machines are made so that the spindle mount is attached to a moving mounting plate that is able to protrude much below the Z assembly:
In this image you can see that this way a zero distance between milling motor clamp and motor axle output is implemented.
You can see a video of a moving spindle mounting cage here (shows Sorotec Alu-Line Z Assembly)
On Onefinity, the distance from mount to collet with a 80 mm spindle that people use is seen here and here.
@Temo I think that if you mill or turn such an adaptor ring I would use the full height of the Euro neck.
Generally I think the problem is the Onefinity Z assembly itself (and this is not the first time I think this way). Anyway I think that a one-piece, dedicated 43 mm mount, machined and offered by Onefinity, would be a better solution than an adaptor ring inside the 80 mm mount.
But I think for myself when the time comes I will see if I simply replace the Z assembly. There are many offered and Onefinity’s rectangular X gantry block should make it easy to attach one.
You can see many third-party Z axis assemblies here: