I read in a book once that a sign of the healthiest way to lose weight is NOT to lose dramatically at first.
In fact, he said you know the body is losing weight healthfully when, if anything it slightly picks up pace, slowly over time, and loses weight slowly but surely, as if in cruise-control mode. I associate it with a purring quality, like a well maintained car, purring, motoring along quietly whilst the passengers enjoy the scenery!. No big shocks and jolts, and minimal side effects, though obviously with a medicine there may be some at the beginning. If side effects are bad or continue for too long, most likely your dose is too high or you’re losing weight too fast.
Also, dramatic weight loss in the first couple of weeks is usually mostly water loss, which is a sign of inflammation and the body retaining water. As you say, which can sometimes be put down to the previous diet.
(Having said that, I do think some people are more sensitive to MJ than others, the super responders. But many are not.)
Also, people forget, in the medical trials that the pharmaceutical companies undertook, the average results of their respondents were fairly MODEST. I think for MJ it was 20% weight loss over a year. For Wegovy/ozempic, it was about 15% over a year.