Confused about maintenance calories per day

So I'm 5'2, well like 5'1 8/9 lol, and started the weight journey in March at 203. I'm down to 146, still considered overweight by BMI but I also have DDD boobs that are ungodly heavy. I want to get to 140 at the lowest, I feel like after 140 I start to look sickly. I go to the gym 5 times a week and do a mixture of 20-30 mins of cardio and 20-30 mins of lifting, plus I'm a teacher so I'm on my feet and moving all day long. I also have hypothyroidism, PCOS, diabetes and hormone irregularities, so a lot of things are working against me lol.

I've been looking up maintenance calories for when I reach my goal weight and it said I should eat 1580 calories to main. I tried it last week just to see and because we were on vacation so it was the perfect week to just try to maintain. Anyway, eating at my maintenance calories made me gain 2 lbs over the week. I thought maintenance calories meant it was to keep you at the weight? I know exactly how much I ate because I track everything that goes in my mouth, so I didn't go over, most days I barely made it to 1500.

I've been eating 1200 a day since March so now I'm worried I'm gonna be stuck at 1200 a day just to maintain even after I reach my goal.