So... my apologies...
Before I went out to buy a new video card, I wanted to 100% make sure my issues were not hardware-related (I had more than I mentioned in this post), so I reinstalled a fresh version of Windows, installed the latest drivers, and everything has been working perfectly. Not sure what made it on my PC over the last 2 and a half years (also going from Nvidia to AMD), but something was corrupted, and a fresh install sorted it out. I had used DDU to uninstall previous drivers, and that still had not solved the issue.
So, sorry AMD, not your fault. Keeping the card, and in the future, when I buy major hardware, I will reinstall the OS.
On a side note, with so many cloud services now (OneDrive, Steam, my dev tools, etc), reinstalling the OS, and having everything back up and running is surprisingly fast.