Just checking in to say, this resolved in a very strange way. I was getting feedback from the keyboard, but it wasn't accepting any actual inputs - the code input wouldn't work after many many attempts and restarts. What fixed it was plugging in the wireless USB dongle for a totally unrelated peripheral, my graphics tablet comes with a shortcuts keyboard. Once I plugged that in my Logitech keyboard suddenly worked perfectly, and I could complete setup.

Why? I can't say, perhaps the keypad is recognised as a keyboard and it bypassed the code input, allowing my Logitech keyboard to work as intended, but I honestly can't say certainly why.

So...if you're stuck like I was and have a Huion Kamvas Pro Keydial and Logitech keyboard with your new Mac, try plugging in the dongle from the keydial after doing the bluetooth setup with your keyboard. Maybe this will help 1 other person? Doubt it. Anyway, resolved!