You could create a custom type based on jsonb type with a trigger to enforce the types inside the jsonb. You could as well write some functions to ease manipulation of such custom type.

There is no out of the box light syntax to do that right now but this could be implemented in a future update/extension. Just look a the complex and successfully implemented geodata types brought in by the PostGis extension.

Your proposal might be more convenient though than creating multiple relations, so we should look into making it just as convenient to create the necessary relations to express this. So lets say your syntax proposal creates multiple relations under the hood - then I'm all aboard!

The "everything is a relation" is a really powerful concept in the relational model. Adding composite column types like arrays, unions etc. are still less powerful than just using relations. But people like it because the syntax is more convenient to use.