Can you advise on weight gain (steroid induced) 3.5 years post gastric sleeve?

Are you still taking steroids? As long as you continue taking them it will be more difficult to lose weight, no matter what you do you will not lose weight, in case you are no longer taking them I would recommend starting a phased diet, reset

Steroids, especially corticosteroids like prednisone or hydrocortisone, can cause significant weight gain even without changes in diet. This happens because they alter the way your body stores fat, slow down your metabolism, and increase insulin resistance. Your body is likely storing more fat instead of burning it, which explains the rapid weight gain.
Your doctor is correct that this is fat rather than water retention. Steroids cause fat to accumulate in specific areas like the abdomen, face, and upper back, a pattern often seen in people with conditions like Cushing’s syndrome, which is caused by excess cortisol. They can also lead to muscle loss, meaning that even if your weight has gone up, a significant part of the gain is from increased fat rather than muscle.
Since your adrenal glands aren’t producing enough cortisol, your body isn’t regulating hormones properly. Cortisol affects metabolism, insulin levels, and fat storage. When you take steroids to compensate, your body responds by storing excess fat much more easily. If you’re still on steroids or haven’t been able to taper off, this could continue.

Now you should do a revisional surgery in the form of gastric bypass surgery. This means cutting down scars of the previous sleeve surgery and converting dleeve to reux en y gastric bypass. Hope that helps