iMac 27 2011 1 led light/GPU replaced

I have scoured the answers here, and while I have found similar posts, mine is a head scratcher.

I picked up a seemingly non working mid 2011 27” iMac with a i7 CPU version. Because it came with its original GPU in a ziploc bag, I knew that the prior owner had it replaced with a “working” model.

Opening up the machine, I could see that he or someone had tried to do it on their own. Screws missing (they were floating on the bottom), or not properly put back. Connections were sound, and nothing frayed.

I replaced the inverter and the original power supply was fine, although I bought another one from a working unit as back up.

The GPU was the 2 GB version for this machine. Fresh thermal paste applied. The screws - the ones with the springs - were slightly misaligned where one was tightened more than the other. I adjusted them so that it was firm but not tight, and the spacing of the heat sink and the card was even.

Put it all back together. Turned it on, got 1 LED light and no chime. Started to unscrew things again. I removed the CD drive, and checked the spacing of the GPU heat sink.

I decided to turn it on (CD player was still connected to the board) and I got a chime and three diagnostic LED lights! Turned it off, and attached the screen. Got a chime and 4 led lights, and the screen came on.

It crashed a few times but when I put it into recovery mode it was fine. Rebooted and then I was back to no chime and 1 LED.

Any thoughts before I consider turning this into a parts piece? Not much money went into this, and I intended this to be a learning experience, but I’m so close to making this function its frustrating.