OK: better news…
I switched from nvidia-driver-430 to the stock Ubuntu display driver in “Software & Updates / Additional Drivers”, and rebooted.
I ran the uninstaller script:
Selected all checkboxes and successfully uninstalled, then rebooted (probably not necessary).
- Switched from stock display driver back to nvidia-driver-430 and rebooted
4K is working again!
I guess the driver gets confused if CUDA 10.1 is installed, but it doesn’t actually get corrupted.
BTW this is running with an RTX 2060 and an LG 4K monitor.