Any update on dotnet core?
Another strategy could be to write classes in a dotnet standard which could be shared by dotnet core(3.1) and dotnet framework (4.7) so your Smartrunner could create unit tests against the common code base assuming its for 4.7 they the same code will also work with core 3.1.
However, there is a load of stuff in C#8 that Typemock just cant produce tests for just yet, so you end up writing 3.1 but thinking like its 4.7 and the results is not ideal.
Maybe Typemock needs another round of GoFundMe so they can fix the dotnet core version as well as create Typemock for Java&Python :)