Luckily I had Backblaze backup set up on our machines. In addition, this older iMac can still be booted up in Target disk mode, i.e. acting like an external drive. So I could salvage all data.

Another funny thing is that Mac’s built in diagnostic mode, after running for good 20 minutes, proclaimed there were no issues with the system. Even though it was clearly failing in the graphics department, even when booting into an installer usb drive (or even a Linux live mode).