I don't have personal experience but have a friend with an itchy cat.
And I have had a couple with food allergies that have caused GI issues, one was on steroids for three and a half years, in the end we lost him but he had other health issues too. Steroids can sometimes cause diabetes (it did not for our cat though) and can sometimes lower their immune system making them more prone to other infections (this was part of the problem for our cat but not the whole story, he had a lot going on).
Are you sure it's definitely fleas that are the issue? My friend did a skin scrape biopsy at the vets for her cat to check it was definitely allergies not something else.
It might be worth discussing with your vet whether it's worth trying a prescription hydrolysed protein food such as Hills z/d or Royal Canin Hypoallergenic. Not al hypoallergenic foods are created equal, they are the best ones to try - if you try this it needs to be this food only, strictly, no treats etc for 6 weeks to see if it makes any difference. If a special diet is a problem because you have two cats you could either feed them both the prescription food (it is expensive though), feed them separately, or buy sureflap microchip feeders so they can eat separately easily (also expensive but a one off expense, that's what we went for).
My friend's itchy cat is on steroids and she is aiming to try and wean off and try something called atopica. I can't tell you much about it but you could have a google and see if you think it's worth asking your vet about it. I think they have to come off the steroids to try it, but ask your vet.
I have also seen something called apoquel used for itchy cats mentioned in cat groups on facebook which I can tell you even less about, but again you might want to look it up or discuss with your vet. It's used for dogs usually but some people use it under vet direction for cats.
I would say form a personal perspective, if the steroids help and nothing else helps, go for quality of life and stick with them. Deal with any problems as they arise - but I am not a vet so they will be able to give you better advice than I can. Wishing you lots of luck.