If you want to lose more than 15% of your pre-tirzepatide weight, I could see why you might want to try to retatrutide instead. Much of the bad side effects of these medicines come from their GLP-1 action. Semaglutide has stronger GLP-1 effects than tirzepatide. Tirzepatide has stronger GLP-1 effects than retatrutide. Also, eating is way more enjoyable without having that feeling that your stomach is full. On reta, I actually desire to eat less even though I enjoy eating more than when on a high dose of tirz.
For cost reasons, it might make sense to combine tirz and reta since you already own the tirz (and tirz is cheaper).
I do think that there is some advantage in terms of safety and knowing what to do with tirzepatide because the stage 3 trials have been conducted, tirzepatide has been FDA approved, and it has already been informally tested in the real world by millions of folks.