Okay, I've got a successful build of the DDR3 SDRAM controller with UniPHY in Quartus 20.1 in Windows 10.
It definitely needs Windows Subsystem for Linux installed - it appears there's no way for the megawizard to work without it, hence why it's not supported on Windows 7. Don't ask me why Linux is a requirement for it - there's nothing there that Windows can't do, so I'll chalk that up to plain obfuscation on the part of the developer.
Although my Windows 10 had WSL installed before, I followed steps 1-3 here (although they were likely unnecessary), but most importantly the megawizard's error log was telling me there was no 'wsl' or 'dos2unix' commands - this could only have been in bash as the error report suggested a "sudo apt-get", so I installed them in bash as well.
I ran the megawizard and it successfully compiled the controller with no errors.
Given the discussion here though, there's no guarantee we'll be able to use it anyway.