Your car is not like a 1972 ford, where there is ONE ‘12 volt’ line, and everything on that line sees 12 v

There are busses with modules that can control sub-systems so that they may not all be at 12, 14 our whatever.

So where ever you dutifully measuring and reporting voltages really are only a shot in the dark. Voltage at the battery, at the DME, at a cigarette lighter may all vary with battery charge, load, etc

I’m gonna guess your FRM is the issue- but just a dumb guess.

I will add that you’ve had issues for two years (TWO FRIGGIN YEARS!!!!). It isn’t the battery.