It looks very nicely organized. Looks like your biggest challenge will be converting UI projects. I even hate to suggest it because it's double the work, but

It looks very nicely organized. Looks like your biggest challenge will be converting UI projects. I even hate to suggest it because it's double the work, but I'd convert all your libraries first to .NET Standard 2. And then make sure they will work fine with your .NET Framework 4.x UI projects. The reason this sucks (I didn't realize it at the time) is that .NET Standard 2 is stuck with C# 7.3. Which means you are missing out on all the C# cool toys. You can't even use .NET Standard 2.1 (and C# 8) because it's not supported by the .NET Framework 4.x. Then go on to the UI projects. Automated conversion may or may not work. Probably not if it's complex. The middleware won't simply convert, particularly if you had routing, filtering, etc... You may have to create new projects. I was able to copy/paste lots of code (routing for instance) but how it was wired was different. If you are successful, you can go back and upgrade the library projects from .NET Standard to .NET Core. Good luck.