OK, so I rebuilt everything from scratch on the INDI side. Fresh Raspberry Pi OS install (Lite 64-bit) and Astroberry repositories set up.
I set up chrony for NTP sync with the UK pool and local servers, and set the gpsd plugin to use the system time so there was no confusion from the GPS. All the clocks in KStars, local system time on both machines, etc all synchronised to UTC and using the same timezones (Etc/UTC).

I used default configurations for everything otherwise in INDI etc.

I left the scope in the home position, pointed at Polaris, and powered up. I then solved a few times and got wildly differing answers. I then moved the scope (slewed from software) and tried again, and again had wildly differing answers while the image remained perfectly still (160 degrees error).