Correct way to generate sources for an external project (with incompatible TargetFramework)?

[lvde0]

I have the following project structure:

Project A

• Generates a JSON file on AfterTargets="PostBuildEvent"

Project B

• References project A via ProjectReference to ensure correct build order
• Consumes the JSON file inside AfterTargets="AfterResolveReferences" BeforeTargets="CoreCompile" and generates source code.

This seems to work. However it requires the TargetFramework of project A to be compatible with project B, even though project B does not rely on any of its code. There are a bunch of issues addressing this problem (See #2661 or #4795), but none of the workarounds described there seem to work.
I am generally wondering if there's some kind of "best practice" for dealing with this code generation scenario?

EDIT: I just found this blog post which uses <UndefineProperties>TargetFramework</UndefineProperties>. It seems to fix the incompatible TargetFramework issue, but it somehow breaks the AfterResolveReferences target. It's called multiple times now for some reason.

EDIT2: After some more testing I made the hack above work. The problem is using multiple TargetFrameworks in the referencing project. Targeting just one framework and including the referenced project with <SkipGetTargetFrameworkProperties>true</SkipGetTargetFrameworkProperties> seems to do the trick. So for anybody struggling with this in the future:

<PropertyGroup>
  <TargetFramework>netstandard2.1</TargetFramework>
</PropertyGroup>


...

<Target Name="GenerateSources" AfterTargets="AfterResolveReferences"  BeforeTargets="CoreCompile" Condition="'$(DesignTimeBuild)' != 'true'">

  <Exec Command="..." />

  <ItemGroup>
	  <Compile Include="Generated\**\*.cs" Exclude="@(Compile)" />
  </ItemGroup>
</Target>

<ItemGroup>
  <ProjectReference Include="..\ProjectToGenerateSourcesFor.csproj">
	  <ReferenceOutputAssembly>false</ReferenceOutputAssembly>
	  <SkipGetTargetFrameworkProperties>true</SkipGetTargetFrameworkProperties>
  </ProjectReference>
</ItemGroup>

This calls a source generator after the referenced project is built and includes generated sources in the compilation. It also works if the referenced project targets a different TargetFramework. I still have no idea if that's the best way to do it though. So would be nice if someone more knowledgeable than me could chime in.