iMac 2011 New SSD and reinstalling OS

I got hold of another machine and extracted its logic board and installed that into the problem machine. The problem still existed, so put the "cooked" logic board back into the machine.

Even though Apple Hardware Test didn't report any hardware issues, next, I took the graphics card from the loan machine and installed that into the problem machine; the machine booted! I could perform internet recovery, boot from USB, boot from other external media, install Ubuntu etc etc.

The problem was with the graphics card!

Now all I need to do is source a replacement card for this farm-machinery...

The ethernet internet recovery starts (spinning globe and progress bar, then grey screen, forever). I've got another Mac and used to DiskMaker to create an installer, El Capitan and High Sierra; both attempts start, then never finish and grey screen. I've also installed El Capitan OS on a USB and try to run the machine from that, but it gets most of the way through the boot up, then stops, a flash of black screen, then grey screen, forever. All the USBs I've made work on a test MacBook Pro I have laying around.

The Apple Genius help said the machine is too old and they don't carry spare parts; a local Apple Service Provider said he thinks it could be the graphics card (I disagree because there's no indication of graphics faults/failing on screen) or the mainboard (at my suggestion - - more likely to be the culprit than the graphics card); in any case he doesn't have spare parts and suggested I "go online".

If the unit is bricked and in need of a new mainboard, I've got nothing to lose from doing a cook? If I botch it, we are still going to need to try and find a replacement board.

I'm not sure how thorough the Apple Hardware Test is, but it doesn't find any fault when (to me) it's apparent there is a hardware problem... somewhere?

Command+V doesn't do anything. There is just a white screen until the OS cannot be found and the folder-with-question-mark appears in the center of the screen.

Other key combinations, such as Command+Option+P+R, or Option, or D work, so it's not the keyboard or not having the keys pressed at the required time.

Has anyone ever had to reflow a iMac main board? I'm thinking this might be the last roll of the dice for this machine?

Ok. The cook at 200C for five minutes had no effect. Anyone got another idea?

For those playing along at home: I cooked the faulty graphics card, 200C for 6 minutes. Works like a charm (for now) -- it probably will die again, but for now, it's working.