I have heard about the use of Interferon Omega in cases of stomatitis, which is often associated with FIV. I don't know that it treats the FIV itself, but it's supposed to help with that mouth inflammation. From what I recall from looking into it around this time in 2024, there was some
promising research regarding it, but my area of the US did not really have it known and/or readily available. Depending on the expense, it may be worth trying.
My own kitty Angus struggled horribly with stomatitis for a while -- steroids were the most helpful in managing it initially, but it just kept persisting every time we upped his doseage. I can't recall all the other medication we trialed. There was something from newer research my vet decided to try us on, more used in dogs I think, and it helped for like a week before the stomatitis came back with a vengeance.
The best known treatment for most stomatitis cases is a full mouth extraction (FME). I know a lot of people are hesitant to make that decision right away, since it's expensive and sounds extreme. I was the same way, and so was my vet. That last medication attempt caused a flare-up with so much pain, he needed opioids to eat at all. That forced our hand and I got him an FME in August or so of 2024. I could tell how much better he felt the moment I picked him up from surgery, he snapped right back to his old self. I kept him on wet food for quite some time after that bc it was easier to give him the gel supplements his orthodontist recommended, but he was able to eat dry food again no problem after a while.

In your shoes, I would get an idea of what an FME might cost you and talk to your vet about that option vs Interferon Omega. If you try Interferon Omega for a bit and it doesn't seem to be moving the bar, I would advise looking at an FME sooner than later. It would have saved my own kitty months of pain, and I regret not going for it sooner.