Mid 2011 iMac 21,5 inch black screen after reboot

So this entire thing started when I launched the opencore application earlier today in the hope that there were some fresh wireless card patches, as the internet on this thing has been acting up ever since I patched it from High Sierra to Sequoia. Luckily that wasn't really inhibiting my workflow, so I didn't really bother

troubleshooting it too extensively at the time. But today when I opened opencore it showed me that there were some fresh patches, so l downloaded them and klicked reboot. All seemed well at first sight. The Mac rebooted with an apple logo and a progress bar, went through the boot picker as usual and suddenly ended up on a black screen.. I think it is worth noting that sometimes the mouse cursor would flash in the top left corner and I could even move it for the split second I could see it on screen. As any sane troubleshooter would do, I rebooted it. Same outcome. Reboot with boot picker key held- white screen and then the boot picker. Then going through EFI boot to macOS sequoia- same result. I did this a couple of times, adding a network in the boot picker too once. Then I stopped my stubborn attempts, did some actual research, and when I got back everything had stopped working. Now when I press the power button the Mac chimes, the screen lights up (in black) and nothing else happens. In a couple of reboot cycles the fans went to full speed, but replugging got rid of that. NVRAM/PRAM or SMC resets didn’t do a thing (though I’m unsure if it recognises the keyboard at this stage, they might well have been empty attempts) and I have no idea what to do next. I have created a bootable high sierra installer in the meantime as I really don’t care about losing data, nothing on there is really important. However, the fact that I cannot access the boot picker is a bit of a problem for a reinstall at this stage…