Your experience with long term weight loss is completely normal. Nearly everyone who loses weight with diet and exercise puts it back on again. I think once the system that regulates appetite and weight gets damaged, it does not really seem to recover in most people, and especially so if the obesity is severe, so to sustain weight loss requires a lower calorie intake than normal due to metabolic adaptation, plus increased hunger due to weight loss. This combo is almost impossible to overcome, and few succeed.
GLP drugs are by a long way the best solution that has ever existed for this problem, with the possible exception of bariatric surgery, which is even more effective, but comes with costs.
So start reta or tirz and stay on it once the weight is lost, and you should only have to lose the weight one last time. And yes you should absolutely use it as a crutch, that is what it is for , the long term treatment of severe obesity. The more you set up losing the weight as some sort of typical diet/exercise program, the more likely it is that it will be unsustainable long term, as if eating less or even exercising is requiring constant mental effort , then eventually people just get worn out from it and give up and put the weight back on. Making sensible better food choices on glp's is a good idea, but thankfully they actually work to do that anyway without any special effort. For long term health good diet and exercise are important, but GLP drugs do cause weight loss regardless of changes in lifestyle. And long term GLP drugs reduce the risks of a large number of serious illnesses, partly due to weight loss and partly as a drug effect, and the worse the obesity, the more important this becomes.