Awfully bad form, but I'm going to answer my own question just in case anyone else comes across this. The problem was interrupts and a shared IRQ17.
To solve, enable MSI in the device.hints file:
You could also use polling, with the following in sysctl.conf:
Adding this to loader.conf (for the 210 over HDMI) finishes the setup:
This assumes your system picks up the nVidia card first. Have a look at dmesg to find out which hdac is which if you leave your on-board or discrete card enabled. Working, chuffed, happy as a pig in the proverbial.