I just tested this on my phone (albeit it is an iPhone 8, so a bit older model perhaps without as many features). A short tap is sufficient to turn it on and off. A longer tap brings up the intensity slider. I was unable to tap the icon quickly enough so as not to register the tap and toggle the flashlight.
Upon retesting a couple times, I did notice that on the first tap the icon is illuminated but it did take a split second more for the actual flashlight to turn on. But waiting the split second was sufficient instead of further tapping.
I’m using the menu that is pulled up while the phone is unlocked (by dragging from the bottom of the screen, on my model), for reference.