Really useful post but I think the first link for the SKF-619 is possibly incorrect, as this is a small ball race for a thin shaft and not a worm gear???
I replaced the main bearings but couldn’t remove the Dec inner spindle bearing, so I just greased it with Lithium grease and left it
I removed all obvious backlash my ensuring the motor gears were fully meshed and fixed the motors hard up to the main gears (instead of relying on the springs to press the worm drive to the gear).
That all said, I couldn’t calibrate the DEC on my Asiair pro It sort of works in RA mode only but still get star trails for 60 sec subs with my Redact51. I was really hoping the the Skywatcher GTI in EQ mode would be the perfect portable setup, but so far it’s been terrible. I got far better results with my IOptron Skyguider Pro, ie several minutes without trails ??