2011 iMac with Nvidia K2100m

Hello!

I recently aquired a deffective 27 inch 2011 iMac. The graphics card was at fault. Before replacing it, I actually managed to get it up an running on MacOS Ventura, using OCLP patches for a Mac mini, in order to avoid the deffective AMD card and use the integrated card.

So i bought and replaced the broken AMD with a Nvidia K2100m. Thing is now I can't get it started at all. Ventura fails to install a gives me an "an erorr occured while preparing the update" message, and Big Sur will fully install, but can't boot. It either waits for a remote debugger connection or freezes at some random lines while booting (always something different).

Any ideas how to get around this? I also tried to make a High Sierra bootable stick on my M1 Macbook, but wasn't able to.