The data gets mapped to JSON to be passed into the template, because ultimately the customizer is creating a dataset, and Vision's serialization isn't really happy with datasets-inside-datasets, so we don't let you create them outside of scripting where we basically can't prevent you from doing so.
I would probably recommend a different approach entirely. Maybe the template parameter is just a parameter name, and then the template instance uses scripting to retrieve that named parameter from the outer template canvas which gets corresponding custom parameters added to it.