In our case, the fundus is the stretchy part that makes up most of the greater, or outer, curvature of the stomach, and is what is removed when a sleeve gastrectomy is done, or is folded up and tied back in a sleeve plication. It looks like with the endoscopic procedure, they suture it up internally to make the fundus inaccessible, leaving the reduced "sleeve" like path open for restricted food flow.
Being a fairly new procedure, one is fighting the learning curve - both for the individual surgeon and for the industry as a whole. The guys that did the first heart transplant were the best in the business, but the patient still only survived a short time; it takes time, practice and experience - both individual and collective - for a procedure to mature into a routine, everyday therapy, so your doc may well be exceptional, but they are all still working out the kinks in a new procedure.
Where to go from here? A bypass was suggested, and this is very common, as most bariatric surgeons were raised on them, so to speak, know them well and tend to be very comfortable with them when things get complicated as they can with some revisions. You note that a SADI has been suggested - was this by the same surgeon, or someone else? If the SADI is a possible, then a regular sleeve gastrectomy should also be workable, as the SADI normally uses the sleeve as its basis, and presumably that would be most attractive to you as that is what is most similar to your originally chosen endo sleeve. Whether you need the malabsorptive component of the bypass or SADI is an individual decision.
When things get complicated like this, it is usually best if one can get a second (or even third) opinion on the problem and possible solutions. There are often several different alternatives available, but individual doctors will prefer, or have more experience, with one over another, while another doc may have different experiences and preferences as to how to approach this problem.
Good luck in working this out...