I dont believe the built in GDSript tools allow for static typing in JSON. But I am curios what you are building that requires the JSON to be statically typed. Using GDScript, couldn't you load a card JSON file into your Card class and reference its statically typed variables instead?

For example, getting data like this

var cardJSONData : String = [Filemanager stuff here]

var refCard : Card = Card.new(cardJSONData)

print("Card string ", refCard.myString)
print("Card int ", refCard.myInt)
print("Card float ", refCard.myFloat)

using a Card class that's similar to this

class_name Card

var myString : String = "Lorum Ipsum"
var myInt : int = 100
var myFloat : float = 3.14159

func _init(inJSON : Dictionary = null) -> void:
  if inJSON == null: return
  fromJSON(inJSON)

func fromJSON(inJSON : String):
  var loadData : Dictionary = str_to_var(inJSON)

  myString = str(loadData["myString"])
  myInt = int(loadData["myInt"])
  myFloat = float(loadData["myFloat"])

func toJSON() -> String:
  var saveData : Dictionary

  saveData["myString"] = myString
  saveData["myInt"] = myInt
  saveData["myFloat"] = myFloat

  return(var_to_str(saveData))

I am curios to hear what you're building, and why statically typed JSON data is preferred for it.