Going on Phentermine is gonna be bad for you if combined with semaglutide.
It's the pretty well known that appetite suppression from semaglutide causes muscle catabolism, that means, a significant amount of weight loss comes from muscle loss which isn't desirable.
Adding a stimulant on top of that would further encourage muscle catabolism as it would increase energy demand, and your body would be filled with stress hormones that's cortisol from reduced calorie intake and epinephrine and norepinephrine from the phenetermine.
Instead of adding a stimulant, it would be much better to add an anabolic (or anti-catabolic) if the goal is improved body composition.