GPU faulty after CPU replacement, mac boots

I replaced the CPU in my 2017 5k iMac with an i7. Everything went fine but now the iMac does not generate any video output. Neither on the internal display nor on an external one. I tried putting the original CPU back into it, same results. I narrowed it down to the GPU because it chimes and boots normally, I can log into my account using the keyboard and activate VoiceOver (which reads the display’s contents out for me), so it definitely still works. However, only 3 (of 5) diagnostic LEDs light up.

The LVDS/DP connector and cable both look fine visually. The display should also be fine. An external monitor does nothing, it doesn’t wake up, no signal, no backlight nothing. So definitely no kind of video output whatsoever. Same for the internal display.

Now my question: I have thorough microsoldering experience and an electronics lab, but have never repaired an Apple iMac before. I neither have a schematic nor a boardview for this iMac model. The ones I found on the internet are too different to find the test points on the board. Is there any other way I can troubleshoot this GPU problem? I’d really like to repair this logic board. Anyone know some points where I could measure some voltages? Thanks in advance!