2011 iMac (12,2) - 5 Vertical Pink Lines & Will Not Fully Boot

I recently pulled my old iMac out of storage as a little side project to see if I could fix it. Service had officially been done on it at the Apple store back in 2015 where they replaced the glass, LCD, and CPU, but these models seem to be notorious for their GPU problems, hence once it gave out, I just stuffed it into storage.

I ordered a 'gently used' graphics card from eBay and installed it. However, once I got inside I forgot I had taken out the HDD years back, so I had to use an old HDD from my prior 2011 MacBook Pro. All this being said, after installation and booting, I make it to my login screen with vertical pink lines. When I try to login, the bar completes about 3/4 the way, then freezes and shows a pink dots pattern throughout.

I would like to assume the graphics card I received is faulty, but curious if anyone else may know something I don't. Is it not able to fully boot due to the graphics card, or is the software on my old HDD potentially causing an issue as well? Should I order a new, but different graphics card that is compatible with this specific model (like a NVIDIA card)? Any troubleshooting I should try first?

Appreciate any insight!