After 2 days stuck on this I had tried all of the above, and then figured it might be something to do with the fact that I installed everything I could on D drive to save space on my SSD… I tried uninstalling UE4 and Visual Studio, reinstalled the latest versions (2017 VS and 4.21 UE4) and let it all install into the default places on C drive.
Everything is working fine, at the cost of 1/4 of my C drive. Thought I’d post in case anyone else tries all the troubleshooting tips and can’t fix it. Still using Perforce on my D, no problems so far (only about 6 hours ago so we’ll see).