I have never met anyone whose waist has grown out of normal growth coming from a lean state. At least from my own time, waist growth happens when insulin resistance is higher, ie you’ve been in an offseason growth phase for quite awhile andd you are pushing food. Stuff starts going sideways. But if you are lean, ie about to rebound up, your waist isn’t going to grow. No chance.
The trick is the fight to maintain insulin sensitivity, maintain appetite, maintain good habits as long as humanly possible. Obviously some structures are more blessed than others, but I’m a walking example right now. Waist growth isn’t happening when I’m lean, it’s happening right now. 30 weeks post show fighting tooth and nail to get food down lol.
Do your vacuums, where your waist trainer, but I wouldn’t stress about waist growth until your body fat is nearing that mental max limit.