Not what we wanted, ay.
Board has been tested with a good PSU, so I think we can rule that out.
You cleaned and reapplied TIM, so, look at the back of the MB, if all four white locking pins are fully locked in place, (all tabs the same length with a black centre pin showing) overheating not likely.
Sadly, it does look like a faulty MB, probable a broken trace somewhere given th pushing down on the RAM, sometimes allowed a Boot.
Apart from using proper workshop testing tools for the MB, I can't think of any more you or I could do to troubleshoot more.
OK, a bit of a reach this one, just in case some sort of CMOS or BIOS loop is happening.
With the PSU switched off, unplug from the wall socket or power strip.
Remove the CMOS battery, > short out pins 6 and 8, holding the screwdriver in place for 10 seconds, > replace the CMOS battery, > reattach the PSU, switch on and short out pins 6 and 8.