I use Marten in my app and would love to see JADNC support it so I have started the work over here:

https://github.com/wayne-o/JsonApiDotNetCore.Marten

While Marten is a documentDb / ES over Postgres it actually can handle relationships. I had a stab at doing the relationships piece but got stuck building the expressions for it - I was using this as a reference;

https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/concepts/expression-trees/how-to-use-expression-trees-to-build-dynamic-queries

I'll need to come back to this when I get time and also add the tests but it is a start :)