All of these API surfaces are pretty large- what is a good way to get a feel for what they do/don't offer? In particular, what are you referring to in .NET Core that you wouldn't want in Framework?
.Net Core supports P/Invoke on Linux and macOS, you aren't going to be using a library that provides bindings for libsystemd.so or whatever on Windows - this is probably one of the most prominent examples. I know there's a handful of others, but I'm having a hard time finding a good article or any relevant documentation on it.
This sounds all good but now we are in the typical Microsoft dilemma when you start something. Core or full? Winforms, UWP or WPF? All very similar but different enough to make a move between them difficult.
.Net Core has API's that aren't applicable to the Windows full-fat Framework, Mono or Xamarin - you don't need or want them there.