I believe I've found a possible solution.
I installed IG 8.6.6.6 on Win10 and it asked me to install .NET 4.8 - so yes, you already had 4.8 installed.
It ran for me, but I've been running IG 8.5.1.22 for some time with no problems.
As a test, I renamed vcomp140.dll in c:\windows\system32, then ran ImageGlass. Up came the "NativeMagickSettings" error!
So I believe a copy of vcomp140.dll is required. I know that a copy of that file is available in the ImageGlass 8.5.1.22 portable download; alas it cannot be found in the 8.6.6.6 downloads.
I can't upload a file here, so I've placed a copy in my Google Drive:Â
vcomp140.dll
If you've already got a copy of vcomp140.dll on your system, then I ain't got a clue! You should be able to put that file either in the ImageGlass install folder or in C:\windows\system32.