No matter how you implement it in the garden, if you want to use it even at colder temperatures, you have to ensure that the water is continuously heated. Otherwise, it cools down faster than you sit inside – especially if the tub is made of cast iron.
So before I would start thinking about the design, I would first clarify the technical implementation. I can't imagine that it will be easy. Or you limit yourselves to using it only as a paddling pool in the summer; the water will then warm up quickly when the sun shines on it, but it requires no technical sophistication.