In short, I bought a donor Mac Pro and swapped parts until I found the culprit - it was the back plane board. i.e. the logic board. I bought a new one for relatively cheap and things worked fine.
Shortly after that the power supply unit (PSU) failed me so I bought a used replacement one and things were back up and running.
I feel like an expert with these machines now. Just wish they could keep pace with the new M1s that will be coming out, because their repairability is actually quite outstanding.
It’s okay that things break after awhile. As long as you can fix it yourself and for a reasonable price. Makes you appreciate them more anyway once you’ve gotten your hands dirty. Just like older vehicles.