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.
I just recently had to replace the logic board on my 4,1 (flashed to 5,1). Tore it down to the minimum specs as per the Apple Tech Guide and as soon as I installed the Processor Tray I was getting a flashing white light on the power. It was either the PSU, the logic board (backpane), or the Processor Tray. I bought another cheap 4,1 Mac Pro that was running well and started replacing parts just one at a time. Only thing that fixed it was replacing the backpane. Unfortunately, there was very little I could do to diagnose this besides replacing the pieces one by one. Thankfully, I can now just order a replacement backpane and then rebuild the “new” Mac Pro and repurpose it or sell it.
Use SuperDuper! to clone a disk with High Sierra to the RAID volume. You cannot install High Sierra directly to a RAID volume. However, if you use SuperDuper! (which is free for this, btw) you can simply copy/clone the drive with the working High Sierra over to your RAID volume. Some important points: (1) When you create your RAID volume, make sure it is in Mac OS Extended and not APFS. (2) You can actually be booted up in the High Sierra install that you’re intending to copy _from_ when you do this. I’ve done this and it works great. Mojave might allow you to install directly to a RAID volume but I couldn’t do that because I was waiting for Nvidia to release their macOS video drivers with support for Mojave.