The memory test is still running... It's at 53% and says it needs another ~24 hours and the total time keeps going up.Flakey motherboard and/or CPU.
I had a clean Windows 10 install that gave me fits on my Dell XPS 13 laptop with all sorts of wonky behavior. Not exactly like this, but all sorts of odd things. Another clean install of the exact same windows 10 version on the XPS 13 and it works fine. So, I'm not quite as skeptical about a clean install potentially fixing the issues on the Ryzen 9 system as I might otherwise be.
Otherwise, I either need to buy a different motherboard or another AM4 CPU for continued troubleshooting since I don't have access to another of either. I'm not certain that using a non Zen 2 CPU will be as useful for troubleshooting as a Zen 2 CPU. It's possible a Zen+ CPU, like the Athlon 3000G might work fine in my X570 board even though the motherboard is bad. Similarly I'm not sure that using a non x570 motherboard will be all that helpful either. It's possible the CPU is defective, but might work fine in a B450 board whereas it fails in a x570 board. They're probably small possibilities, but would prevent the troubleshooting steps from being as conclusive as they could be.
I suppose the most conclusive things would be to buy another of the exact same hardware for swapping, but even that has a potential coverage miss as maybe it's an issue with the specific motherboard I have and the other hardware I've put it in and another x570 board wouldn't have the issue.