There are a few things you can rely on during this process. And all of them are okay. None are wrong or right.
Everyone loses at their own rate. Some lose more quickly. Others lose more slowly. Many factors influence this including, age, gender, pre existing health conditions, medications, etc. Hence the range of possible loss your surgeon suggested in your first month.
The more you have to lose, the faster you’ll lose at first.
Everyone’s rate of loss slows as they get closer to their final weight.
Stalls happen. You will have times the scales won’t move. The first often occurs around week 3+/- & they often last 1-3 weeks. Just stick to your plan & your weight loss will start again.
Not every one reaches their goal & some exceed their goal. The average loss at the three year mark is about 65% of the weight you were to lose to put you in a healthy range. I’d expect that is where the 55lb weight loss your surgeon has suggested comes from.
Like you, my BMI was 35. I’d lost all my weight by 6 months but continued to lose for another 11 months while sitting out my maintenance, albeit very slowly, to about where I am now. But that was my journey. Yours could be similar or different & that’s okay.
All the best with your surgery & journey.