Great news! I also have SLE (diagnosed 2003) and Fibromyalgia (diagnosed 2001).
You are blessed, as I was, to find a surgeon who would perform the procedure. especially since you have multiple autoimmune issues. A lot of surgeons won't touch a lupus patient for elective surgery!
My surgeon kept me in the hospital for a week to make sure the trauma of the surgery wouldn't cause a massive flare. My weight gain was helped along by long term prednisone. Highest was 405 pounds, surgery day was 397.
I'm now 5 pounds below my goal weight of 160. my weight had been stable between 170 and 180,taking into account about 15 - 20 pounds of loose skin.
As you get further out, you may find it a challenge to keep your weight stable, meaning not losing weight unintentionally. I'm in awe of the calories I consume, thogh calorie counting is not part of the DS repertoir, I was just curious one day. I added up everything in my food log, and it came out just under 4000 calories! With *no* junk food!
Be well, DS lupus sister, and count your "spoons" carefully! (Google - The Spoon Theory)
My first severe flare post-op happened a couple weeks ago, and I had to take a prednisone taper for the first time in a long time. I was down for about a week and a half. I had a mini flare with fever in August, the week before classes started. Lots of stress in my life these days, and that's undoubtedly what brought on the flares, along with the change in the weather.