I worked on a Pinball Pool for a customer that exhibited all of the symptoms yours is. I tried everything you have and more.

I either replaced the CMOS or made a recommended modification to the power circuit, maybe both.

I modified the reset section in case that was preventing it from booting.

I had replaced the power supply and it was spot on.

The MPU was perfect with no sign of physical damage or battery corrosion. Ultimately, I gave up and ordered a NiWumpf. If you get yours figured out let me know, I still have the original MPU.

I believe that’s the upgrade I did and it made no difference. I also did the ground mod.