The purpose of the threaded collar is to preload the bearings. The spindle runs silky smooth with zero play. The bearings are so big, they will literally last indefinately in this application.
I don't understand the purpose of a sleeve, unless you are suggesting a 'crush' sleeve as used for example in older independent rear suspensions (Corvette, etc). Those are to maintain a specific clearance as opposed to preload.
The reason I chose an open frame instead of a solid block is the difference in expansion between the steel spindle and the aluminum head. I rely on the 'spring' of the open frame to accommodate the differential without binding the bearings. I considered a steel or cast iron solid block, but I decided against that because of the extra mass it would add that has to be moved around by the little steppers on the table. Ideal would be a hollow cast iron box similar to the x2 head, but I have no way to make such an item. I did consider using an X2 head casting. The open frame also provides more mounting options. With slotted mounting holes, I can mount the 4th axis anywhere from 0 to 45 degrees on the table. although there are keyways for quick alignment with the x axis.
I've had the motor up to 3,500 RPM so far at 62V. In service, it will run at 72V, but with a 1 ohm braking resistor, so I'm not sure yet what the top speed will be. Without the resistor, it calculates to almost 4,600RPM at 72V. I'm looking for 4,000.