hi mummy! i to am 6.5m pp and missing my nice bum too!!! i read that this is due to our body's misalignment during pregnancy that we carry over when we give birth. so work on your alignment first. be conscious of your posture and make sure that when you stand, stick your butt out like there's a string pulling it upwards :). after fixing your posture, do stretching first then the squats and lunges. cos if you don't fix your posture, you won't achieve the best results even if you do squats and lunges. all the best!
My favourite workout is doing the bridge because I dont want thicker thighs lol. It focuses on the bum and abs. It depends on how often and how many reps you do. I can feel the tightness in 1 mth but of course, depends on every body.