So I installed .NET 2.1.300-rc1. I can now build projects that target netcoreapp2.1 but changing the .csproj's <TargetFramework>netstandard2.0</TargetFramework> to have netstandard2.1 gives the following error ...

The current .NET SDK does not support targeting .NET Standard 2.1. Either target .NET Standard 2.0 or lower, or use a version of the .NET SDK that supports .NET Standard 2.1. C:\Program Files\dotnet\sdk\2.1.300-rc1-008673\Sdks\Microsoft.NET.Sdk\targets\Microsoft.NET.TargetFrameworkInference.targets

Please MSFT ... don’t make net core, net standard and net framework versioning more complex than it needs to be.