I'd almost suggest some kind of custom rotary head.

A rotary table seems like overkill for machining plastic, but is probably the easier option.

I'd think a more basic axle with clamping head, driven by either a gear reduction stepper or double belt reduction would provide enough torque/accuracy, and be far quicker at spinning the pipe.

The clamping head/chuck could be a challenge, as you can't really put too much pressure onto PVC pipe. Perhaps some sort of metal plug for each pipe size bolted to the rotary head to support the inside of the pipe and prevent it collapsing in, then jaws which clamp down onto the outside. The big issue is covering the range of pipe sizes, while not making it too hard to swap between sizes.
Perhaps have the centre plug held on with a centre bolt, then jaws, or even a big pipe clamp to clamp the pipe onto the centre plug.