Greetings and welcome to a fellow IBD/Diabetic cat mom! I have one of those myself!
My cat Riley was just diagnosed with diabetes he was diagnosed with IBD 6 months ago and has been on a steroid shot depo. He gets that every 30 days and the vet said he believes that the steroids is what caused the diabetes.
Skooter was dx'd with IBD several years ago and the diabetes was last April. My vet also thinks the steroids are what did him in, though he was not always on the steroid. Once he was dx'd with diabetes, we stopped the pred and actually changed to budesonide, so I would check with your vet and see if that is an option. I get my compounded as a liquid and shipped to me. He gets the chicken flavor, but make sure there is no sugar in the compound and/or flavoring they are using as that will cause the BG to jump up for sure.
Now I'm concerned with all the medication he's taking on top of all these health issues. Right now he's taking Baytril one milliliter daily for 21 days for UTI. And he's been taking for the past 6 months 1 milliliter twice a week of azrithromycin.
There are others far smarter than I on this topic, however I would wonder if they did a urine culture to see what kind of bacteria they are dealing with to make sure they are using the most appropriate antibiotic for him. It is possible that he may have built up a tolerance to the azrithromycin and it may not be effective. Again, I am not great with advice on antibiotics, so I will leave that one for others to ponder. It's not a bad idea to take stock of what meds he is on and see if they are the best with the diabetes dx, there may be alternatives that you could use instead. But sometimes it comes down to treating the other conditions first and then working the diabetes treatment in with that. But again, I leave this kind of medication stuff to the "pros."
I did have him on a probiotic as well I stopped giving it to him 3 weeks ago because the doctor wasn't sure if it was ok to take with diabetes although the antibiotic has sugar in it!!! Doesn't make any sense.
Skooter takes a probiotic. He gets half a pill in the second meal of each insulin feeding cycle (every 12 hours).
Nexbiotic was what was recommended to me by the FB IBD support group I belong to, many of them also have experience with diabetes as well. When they made the recommendation, they were aware of his diabetes as well. Knock on wood, I honestly couldn't tell you the last time Skooter had a flare (which normally involves vomit).
A little additional info on my guy

He is 18, he was on a rx dry food and up to 5 units of insulin twice a day when we were getting started. With the help of everyone here, I started to transition him SLOWLY to a wet food (didn't want to upset the IBD!). When we were doing 1/2 and 1/2 of wet and dry, we went to 4 units twice a day. When he was fully transitioned to wet we dropped to 2 units a day...the full transition happened last July. Today, he is on a DROP of insulin (0.10) twice a day with beautiful numbers almost always in the normal range of 50 - 120. His insulin of choice is Lantus and he eats Nature's Variety Chicken. Eventually I would like to get him on a commercial raw diet, but life is really crazy right now, so that will have to wait.
I started giving him his probiotics back yesterday because I noticed he was constipated and I just gave him some pure pumpkin hopefully to help.
Skooter recently went through a bad bout of constipation and would not touch the pumpkin or squash. He actually ended up with two enemas to get cleared out. He is now a lifer on 1/8 teaspoon of miralax, given in the first meal of each insulin food cycle. I would definitely check with your vet before starting miralax, Skooter started it based on an ER vet recommendation and when we had our follow up appointment with is regular vet we decided to keep him on it.
So I have probably overwhelmed you with info as it is, but just know there is a WEALTH of information on this board. Ask as many questions as you need, ask for clarification or something explained a different way if need be.