You should have a doctor who can give you good recommendations. If your doctor is fat, then they clearly don’t know/can’t follow their own guidelines, so treat them with a grain (block) of salt in that case; you’ll need to eat only what you need to sustain your healthy bodyweight at your activity level (without nutrient deficiencies.)
BTW, It’s still possible to get gallstones without a gall bladder, or so we were told. Have they checked for that? Usually it’s an ultrasound or a CAT or PET scan. Those latter can be expensive.
I saw elsewhere that you were using Soylent 2.0, which ought to be perfectly safe as part of a reduced calorie diet.
Your two levers are your diet and your activity levels. From your description I am assuming you’re sedentary (as are most people who work in desk jobs nowadays.)
You will need to, not necessarily EXERCISE, but become non-sedentary. Being sedentary is a major part of our collective problem with not burning calories. If you’re able to walk for 30 minutes a day, then do so – ten minutes at a time if necessary. If you’re trying to find exercises that won’t aggravate arthritis, then demand that your doctor recommend a properly skilled physical therapist who can teach you some exercises that will work. Even standing more than you sit is a good thing.
Then comes controlling the diet.
Do not do one of the super-low-calorie fasting diets. Those used to be prescribed to people who couldn’t gain weight and were unhealthily skinny. You could do that with Soylent but it wouldn’t be a wise thing to do.
Instead, figure out your lean body weight – there are electro-capacitance scales that do a good enough job – and then figure out what your ‘optimal’ weight would be based on that. E.g. a woman with a lean body mass of 120 pounds and an 18 percent body fat (average) should weigh about 145 pounds (120 = 0.82n -> 120/0.82 = n -> 146 pounds.)
Set your targeted daily calories based on that bodyweight and it will automatically contain a deficit. Of course, you might need to eat more than that at first to keep from losing too fast, which is as bad as not losing.
You might want to try the ‘limited fast’ approach given elsewhere in this thread, or you might want to try the ‘no sugar spikes’ method where you don’t ever let your blood sugars drop too far from hunger (and thus don’t stress the pancreas.) Your doctor, and your own body, should be able to tell you if this will work.
And the key is being consistent.