Hi - difficult decision for you. If it were me I would get the mastectomy over with sooner than later, as there is a chance the DCIS could turn into invasive, depending on how aggressive the cancer is. Cancer is life threatening at any stage, whereas obesity is not, in the short term. Once you get over the mastectomy, then you can schedule the bariatric by-pass surgery.
I am not an expert on alternative dietary regimes to prevent a bc recurrence, and each individual has the right to make their own decisions whether to go the alternative way, or have tried and tested allopathic treatment, e.g. chemo, radiotherapy and hormone treatment. I chose allopathic medicine, and am on Arimidex to prevent a bc recurrence (I had an invasive tumour, lymph node involvement as well as DCIS, so wanted to do whatever my surgeon and Oncologist suggested.) Your diagnosis/prognosis is very good compared to a lot of people dx with bc, and you probably will not need chemo or radiotherapy. Certainly, reducing your weight will help somewhat, as will a good healthy diet with 5 portions of fruit and vegetables a day, oily fish, and only 2 or 3 portions of red meat a week. I eat more chicken and fish since my diagnosis and rarely eat red meat. A low fat diet is recommended for the vast majority of people, with or without cancer.
Perhaps you can talk to your bc surgeon as to the risks involved in delaying the mastectomy for his expert opinion. We are not experts on this forum, just people who have had bc, or had relatives/friends with it, and can therefore only give you the benefit of our on experience - and some cyber hugs!
Hope you get the medical advice you need, and do well.