I've solved this and thought I'd post it for anyone interested.
By the way, this aircraft is still being set up and hasn't flown yet, so there are numerous settings that aren't yet configured.
Here's what I found:
After reviewing logs, I confirmed that the autopilot was knowingly making the commands causing the twitches, so it wasn't a hardware fault or interference etc.
Further, in playback, I could see that RTL was activated at all times during the crazy twitching.
Manual was actually selected on the transmitter, so I’m not sure why it was in RTL, but anyway...
And I confirmed now that the same behaviour happens if I select RTL.
Just now, the switch selection of Manual was being obeyed and there was no crazy twitching.
My conclusion: it was caused by it being in RTL mode.
— why it was going into RTL
— why RTL would cause such extreme maxed-out movements.
Anyway, things are looking up!