820-3437-B power off after power on

If it is not an issue with the SSD you are booting into (boot into a known good external SSD too), this is going to be a nightmare to solve.

Check all areas of the board to see where the original liquid damage was. PP1V05_S0 issues can cause this to turn off mid boot; I have also seen these with shorts inside the board around this circuit, which causes this problem (but you will smell burning if this is the case).

Have you ultrasonic cleaned it? Could be corrosion under something which was not cleaned before. Has it had any work carried out to the board?

If it is not the SSD or something obvious (again, checking for where the original damage was), it is going to be an unfixable issue or very time consuming to find out the cause, and I would not recommend repairing it if this is the case.