[avandonk]

Any mount whose motors have a gearbox are inherently going to give you grief even when autoguided due to the random micro jumps produced by the gears. Never mind PEC.
I replaced the motors on my HEQ5 with Astromeccanica motors. These are belt drive and are microstepped. I have no backlash on either axis on a standard out of the box HEQ5. The tracking is superb.
I am not interested in goto as I think it puts to much load on the mechanicals especially when near the 'limit'. The astromeccanica motors will give you goto from a standard HEQ5 or EQ6. I found it too painful to use as I can generally find what I want almost instantly without wasting time slewing etc.

Here is a crop from a 15minute autoguided exposure at 1800mm F9 JPG straight off the camera. Image scale is one about one (0.94) second of arc per pixel.
This was taken with a TAL 200K and all up weight of the optics was 13.5 KG.

This is one alternative it may not be correct for you. For astrophotography at any focal length though it works very well and is portable.

The image is of the core of Omega Cent. The EXIF data is there apart from the focal length.

Bert