@o_lampe it wasn’t, as it happens. I redid the firmware from scratch using the RRF configuration tool which gave a slightly different setup to the one I was working on when I asked the spindle question.
Ironically having fixed the issues I’m back to essentially my original spindle config.
What I’ve ended up doing to solve that original issue is break out heater3 (and 4 for the laser) and use those to PWM drive an external MOS chip that level shifts the PWM to a breakout from the fan 5V… which is then the right level and “side” of the spindle controller to control it properly!
It seemed to work when I tried it at 9:00 last night but that’s my cutoff time for trusting my brain not to do something stupid so I’ll be doing more tests and checks this afternoon.