Postpartum weight loss

Heres a bit of a back story. Back in 2015 I made a decision to get healthy. I was 285 lbs I'm 5' 7" by the way. I started running and joined a gym got very committed and was exercising 5 days a week. In about 9 months I lost 55 lbs I got down to 230 lbs. I met my fiance and kinds put going to the gym on the back burner after we moved in together. I got pregnant a few months later and just stopped exercising all together I gained about 25 lbs with my pregnancy in 2016. After I had my daughter I lost it all pretty quickly because of breastfeeding but over the years gained most of the back. I just had my second baby 2 weeks ago. I was about 265lbs when I got pregnant this time around. I was 294lbs at the end of my pregnancy. I'm now 16 days PP and 271.4lbs so I'm losing it pretty quickly again. Once I get cleared at my post partum checkup I would like to get back into exercising again. Ideally I would like I get down below 200lbs. Back in high school senior year I was around 175lbs I'm 28 now been out of school for 10years. According to BMI chart I should be 136-140lbs for my age and height. Is this even an achievable weight goal or is 140 too low? I know weight is really just a number but my goal is to be an a healthy weight range and adapt healthy habits into my lifestyle again so I can play with my girls and set good eating and exercise habits for them. Also exercise helped a lot with my depression and I've been struggling with my depression again for the last few years since I stopped exercising.