You might be able to get this approach to work using the transform profile. All your Items would be linked to the one Channel and you’d apply a custom transform profile on the link to each individual Item.
However, the “correct” way to model this would be one Channel per value. It’s frankly no less work to add a transformation per link and then if your JSON ever changes, you’ll have a bunch of places to fix it instead of one central place to fix it. It’s the job of the Thing to handle implementation details like extracting the values from a JSON String where possible. The Items shouldn’t care what format the data came in to begin with.
@Bassdriver describes the easiest way to handle this by creating the separate Channels.