Maybe I didn't get your point correctly but this seem actually doable in PostgreSQL.

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.