Cat has allergies that don't get any better

I have a 7 year old manx cat who developed allergies in roughly the last year. I first noticed what looked like a big hot spot on her back, so I took her to the vet (a big animal hospital in my town in the Chicago suburbs) and they hardly even looked at it - just shaved/cleaned the spot and put her in a cone. Didn't give me any advice on what it might be or what could have caused it. After the two weeks with the cone, the spot had healed but my cat licked it open again almost immediately. I ended up taking her to another vet closer to home that did a much more thorough examination on her. They scraped the spot to look for mites, they shaved/cleaned it, checked her ears, took her temperature, etc... and determined it is allergies, because my cat also had a bump on her lips. They gave her an allergy shot which stopped her itching, and cleared up all her spots/lip bump in what seemed like a day or two. After about 3 weeks, the shot wore off and my cat opened up new wounds itching her back. I then took her back to the vet and they gave her another shot, and gave me a cream that I can put in her ears once a day to help her allergy symptoms. This seems to work but my concern is that the vet told me that doing the shots/allergy medicine long term is not a great idea because they can lead to kidney/liver failure and increase the risk of diabetes (my cat is overweight as a result of being manx, so already higher risk for diabetes).

She only developed these allergies about a year ago, and we hadn't changed any part of her routine so I'm unsure what she could suddenly be allergic to. She also seems to have them year round instead of having an allergy season. What do you suggest I do now? Should I keep doing the allergy meds for her despite the risk of kidney/liver disease and diabetes?