What to consider when designing a pcb for HV?
The potentially (no pun) biggest problem as I see it is breakdown (or "tracking") of the high voltage across the surface of the PCB and I'd certainly consider making slots in the PCB to make the effective surface distance from HV copper to LV copper as big as you are able.

Then, around sensitive LV circuits (that may be close) to the HV stuff I'd consider a protective grounded guard ring to prevent potential arcing to the LV circuits and their destruction.

It's a dangerous voltage level so keep safe.