If you want to use just the double tap gesture, have a look at Settings > Accessibility > Quick Actions instead of the more full-featured AssistiveTouch.
For details, see “Use quick actions” on the Use AssistiveTouch on Apple Watch page in the User Guide. (Oddly enough, for that setting, the gesture is called “double pinch,” but it’s the exact same finger motion.)
That feature is surprisingly reliably, even though I have a now-older series 6 watch. There is one exception, though: the gesture consistently fails to dismiss alarms from the Alarms and Timer applications.
The UI does show the highlight outline around the respective button, and the little Quick Action info panel appears at the top of the screen. That little panel disappears right after performing the double tap/pinch, so the device must register the gesture. And yet, the alarm just keeps going, no matter how often I double tap/pinch…
Does anyone else here notice this issue?