How long does it light up? Because I tested this on a functional device, it does the pattern (LED, vibration, screen) I described, on a dead core-module it lights up RED half a second, and on a bricked device it lights up RED for around 30 seconds (only saying this for reference)
So your core-module must be affected… This is the test I found some time ago on a FP support page.
Well… I opened up quite a few core-modules, and I wouldn’t desolder anything without professional soldering equipment and experience in microelectronics. If this is your case, you may want to try, but I doubt anyone would be able to tell you what to do, since the schematics aren’t open, and a smartphone’s motherboard is reaaally complicated. I’m not even sure it’s possible to know where the problem comes from in the motherboard.
Now, a last thing you might want to try, but it has very little chance of succeeding and I’m not sure you tested everything else you could (though I’m out of ideas)…
You could try powering your phone in EDL mode. Check the topic I started on Fairphone 2 - Booting in EDL mode and unbricking. You will see how to enter EDL mode by shorting the two test pins. Once this is done, find a way to unbrick your phone with edl, I haven’t succeeded yet (though I may try again soon, can’t say).
In any case, good luck!