Bluetooth uses very very little power. Think about it. While I don't know the specifics of the iphones BT unit, let's take the Airpods and assume it's similar . They have about a 25mah battery per earpiece (from what i gather from a 3 second search) and they last about 5 hours. So 100mah per 20 hours. Your iphone, let's say, has 3000mah battery. So in 20 hours BT would use 100/3000 pct of your battery in that period. And that's actively using it.
It's really not worth the effort to bother toggling it.
Now, if you still want to, well that's what automations are for.