.NET standard isn’t the biggest issue with making source generators. You can’t add dependencies to your project, which is an absolutely huge oversight IMO.

You can add dependencies, it's just important, that you also add the dependencies to the project where the source generator is used:

SourceGenerator.csproj:

  <ProjectReference Include="..\Dependency.csproj" />
FinalProject.csproj:

  <ProjectReference Include="..\Dependency.csproj" OutputItemType="Analyzer" ReferenceOutputAssembly="false" />
  <ProjectReference Include="..\SourceGenerator.csproj" OutputItemType="Analyzer" ReferenceOutputAssembly="false" />
And I also recommend to add "<LangVersion>latest</LangVersion>" to SourceGenerator.csproj to use new features like raw string literals.