Thanks Jazz, that is very helpful.

I retrammed and recalibrated the X and Y/A axes, and could find no discernible backlash. It is basically an Avid replica with their Rack and Pinion drives. Perhaps my measuring tools are just not accurate enough, but I managed to get an acceptable result by adding 0.15 mm backlash compensation to the X with a compensatory acceleration of 20% extra (600mm/s/s/).
I set the max Velocity at 5000mm/m and Acceleration at 500mm/s/s on a small item profile.

The maximum before lost steps is actually 21000mm/m at 1000mm/s/s acceleration, but I would have to bolt the table to the floor to run at anything like those speeds with a gantry of around 80 or 90kg total. Seriously powerful motors those Lichuan Nema 23/24s. Thanks for the recommend on those btw.

The OP on that forum with the Out Of Round problem was confusing the terminology, as he linked to NEMA 34 3 Phase 12NM Closed Loop Stepper Servo Motor Driver Kit he said he was using, not true Servos. His solution was:

"I was right about it not being backlash or anything loose in the system. I went through all the settings on the drives and I found a setting for speed smoothness.. This was set at 8 on the X-axis and 2 on the Y-axis.
The consequence of this difference between the two settings was the 0.5mm OOR on all circles. When adjusted to 8 there was a pretty aggressive speed smoothing going on.
I adjusted both drives to 2 and behold, perfectly round holes and even sized diagonals in squares."

I ran the debug software on the drives and all settings are identical, just with the exception of the Current Loop Proportional and Integral Gain, which I see is set by the drive automatically according to current fluctuations I take it. The Position Loop Proportional Gain, although I take your point re servos v closed loop steppers, is set way above the default, at 2500 v default 600. The Speed Loop Feedforward is way lower than the default at 35 v 120 and Position Tracking Error Threshold is at 4000 v 400 default.

I surmise that I could play with adjusting the Speed Loop Feedforward or other settings but I don't have enough knowledge on this, and how they relate to IJK commands on steppers v servos, so I will leave that for now, after all I am not milling parts for the space shuttle.