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

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 https://github.com/dotnet/msbuild/issues/2661 or https://github.com/dotnet/msbuild/issues/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](https://jaylee.org/archive/2019/01/25/create-a-buildreference-dependency-between-sdk-style-projects.html) which uses `TargetFramework`. 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 `true` seems to do the trick. So for anybody struggling with this in the future:

```xml

netstandard2.1

...

false
true

```

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.