.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.
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.