This looks like a faulty graphics chip on the logic board. You have 3 options to repair this:
- 1- Replace the entire logic board.
- 3- What i have done that works best is to perform a BGA reballing on the graphics chip but you need specialized tools and skills with soldering to do this and can be more expensive than replacing the logic board
What I would do in your situation is attempt the reflow first to see if it works. If it doesn't then I would just replace the logic board. See this guide: iMac Intel 17" Logic Board Replacement
If you want more info on reflowing or reballing try a google search of GPU reflow diy, BGA rework.
I hope this helps, good luck!
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