Horizontal black lines covering bottom 3rd of 2021 M1 iMac screen

Purchased 24" M1 iMac in late Aug/2021, in New Zealand.

Generally keep brightness fairly low (which is why this problem probably hit after 3 years, not the usual two).

Black lines appeared 04/Oct/2024. See attached pic.

Hardware problem not software (screenshot doesn't show lines, photo does). Reboot/reinstalls etc don't fix. External screen works fine.

Definitely related to: same problem, same problem, same problem, and, well, probably most of these: Apple forum search on "horizontal black lines" - showing 183+ results