Service call “Climate: Set target temperature” should accept helper as value for the target temperature. Now it only accepts explicit numbers, see screenshot of my attempt to create an automation (the helper in the screenshot is of type “Number”):
There’s not a single option anywhere that supports an entity_id or a template; it’s one or the other. The advantage of a template is that it’s far more capable (for example, it allows for calculations such as setting different values depending on time, presence, etc).