your butt shape will change after all the weight loss. Mine was pretty saggy - but I had a lower body lift which tightened it up. Usually lower body exercises like squats and lunges will help with glutes - but again, your shape will change pretty fast, and there's not a lot you can do to help the sagginess with the amount of weight we typically lose.
immediately after surgery, all I was cleared for was walking. After four weeks, I was allowed to do everything except for weights. At eight weeks out, I was cleared for weights. The first few months I did mostly water fitness classes - and walking. I also did the stationary bikes at my gym. Now I do dance cardio (like Zumba) and biking, but I was too heavy to do those when I first started on my journey.