After many unhappy rounds of installing and uninstalling CUDA drivers (11.1, then 10.3, then 11.2), updating Windows 10 with no joy (nvidia-smi kept showing 10.3 CUDA was installed even after uninstalling it and re-installing!), and switching card slots.
I saw in NVIDIA Control Panel | System Information | Components that the NVCUDA.DLL was 10.3, not 11.2. I then uninstalled all Nvidia apps, but the NVCUDA.DLL for v10.3 remained. Then it dawned on me to search the system for NVCUDA.DLL. DLL Hell. I then manually removed all Nvidia DLLs from C:\Windows\System32 and C:\Windows\SysWOW64 , rebooted again, installed cuda_11.2.0_460.89_win10.exe as admin, and voila: nvidia-smi now reports cuda 11.2 and deviceQuery detected the CUDA driver, and my Emgu C# app under development now finds CUDA!
I’m not sure why the installer could not detect and remove the older Nvidia DLLs. But this worked so I am happy.