Please DO NOT add surgery. That sounds horrific and destroys the vibe. I would prefer to avoid this horror game feature.
Different medicine types should be enough (dozens of pill and injection options exist). But if you really want to add surgery, then there must be some serious warning signs also included like "permanently disable gaining ability for that citizen and cannot be undone", which show that there is no way back, and also terminates the model career as followers would not like it.
Instead you can add some real life health maintenance options which are very common among heavy people like high blood preassure (can be treated with pills), and diabetes (can be treated with insulin). These don't have weight loss effect, but would help to keep up good health conditions and good mood even at USSBBW stage. (These could be daily routine and/or doctor's visit gameplay options)