Greetings,
and welcome o this Apple (user-to-user) Commnity!
Re: "AssistiveTouch Custom Gestures missing on iPhone 15"
Wondering whether Apple support's article (link below) can help: (Or maybe this is just what you tried already ?)
Use AssistiveTouch on iPhone - Apple Support
scrolling down the article to look under "Create custom gestures"
Eg: says:
"To use your custom gesture, tap the AssistiveTouch menu button , tap Custom, then choose the gesture. When the blue circles representing your gesture appear, drag them to where you want to use the gesture, then release."
All the best :-)