I can totall sympathise as my cat Tinka has had skin allergies for years, we are still not sure if its food, environment or both. Tinka has been on predisilone off and on for years and I believe its what led to her diabetes diagnosis this past December. When we took her off the pred we saw a huge improvement to her BG! Only problem is her skin got worse. So we temporarily put her back on, then we added in apoqeal then slowing weaning off the pred. To be honest I'm not sure how effective it is yet as she's still on 1/8 steroid. We will know more once she's off the pred. I can keep you posted on that. I haven't noted any side effects though. I dont like drugs in general as they tend to all have a pay off, but at least it doesn't affect the BG. Apparently it suppressed one part of the immune system rather than the whole immune system like the pred.
For us though I think that one of the things that is helping Tinka is the raw food diet. Her weight is improved so much and its so LC its helping her diabetes so much. We are also doing homeopathic remedies to help support her immune system. Suppressing the immune system with drugs is really only a temporary measure because each time we stop the skin itch, hot sport, biting etc just comes back. I do believe it could also be linked to a compromised gut and I believe this si from years of dry food diet - science plan and royal canine, its when all the skin issues started up. I think stress can play a role too. The one vet we working with recommended the raw food diet, I wasn't sure Tinka would like it as she's pretty picky but I'm amazed how much she loves it! Where about are you based? Im in Canada. We have feed both home prepared raw, as well as prepared store bought - we use Spring Meadows - the rabbit and pork she absolutely loves. I'm on the facebook group "Cats completely raw and proud" and that has lots of info on how to balance meals or buy brands that are balanced with meat, organ and bone ratio. I was a little concerned with auto-immune to be dealing with raw but we have all been healthy with no issues since going raw. We are currently trying to find one protein that we can stick with for 8 weeks to do a food elimination trial however she gets bored of food and likes variety so we've not managed that yet. Obviously they need to be eating well to shoot. But if you can try something like that maybe that would help. My vet supports a raw food diet, but is more drug focused rather than finding the root cause. The other vet I started working with is a regular trained vet but more holistic minded and he works online. He is the one that is prescribing the homeopathics, so we will see, if we manage to get her off the drugs if the skin issues resolve or not. He only advocates for a raw food diet as he says many cats and dogs get food allergies because they can't digest the proteins especially if they have gut issues. Cats are literally made to eat raw food, there should be no reason to use antiobiotcs when feeding raw especially if you are buying from a reputable company or making your own.
I know how testing dealing with allergies is, its such a stress not knowing whats wrong. I really hope what ever path you choose that you see improvements soon! Keep in touch with the progress and I'll do the same! All the best