Hey Roger,
Do you still have the 65 mm mount installed?
The router or 65 mm spindle that is slid that much downwards into the original 65 mm mount is already a source of adding more leverage force to the system. If you look at professional milling motors, they are always clamped near the bottom where the axle exits the housing. Now instead of adding a second instance of increasing leverage force into the mechanical system as Riser Blocks do, I would rather lift the router / spindle so that it is clamped more at axle end. As you correctly stated, the Z stepper motor is in the way and router / 65 mm spindle would bump the stepper cage. But this problem can be solved by lifting the Z stepper cage using taller stepper mounting blocks (see pic 2) and extending stepper axle with another coupler. The advantage over Riser Blocks between X and Y axes is that 1. you do not add a source of increasing leverage force somewhere in the mechanical system, and 2. you eliminate a source of high leverage force that is present in the original system. The difference is, when you lift the stepper cage, you do not interfere in the stability of the mechanical system between your milling, plunging and feeding bit and the machine base.