You make up a slippery elm bark gel by mixing 1 tsp with 1/2 cup cold water using a fork or whisk then put on a very low heat and continue stirring with fork or whisk for a minute or three until it thickens a bit. Put it in a jar in the fridge and it will last 5-7 days.
I give it to Vyktor specifically for his stomatitis he has it before his afternoon feeds but not the morning ones because that has other supplements and SEB can inhibit absorption of other meds - you wouldn't want to give it for a couple of hours either side of ABs. It works miracles sometimes, i.e. He won't eat, give the SEB try again in 15-20 mins and he'll eat - I syringe about 2.5ml at a time and he'll end up having 2-4 of those a day.
I was making it up with chamomile tea (German chamomile flowers) because of their pain killing and anti inflammatory properties but have recently stopped because it looks like he might need some further dental work - he has three teeth left, the vet (dammit) left them because at the time they weren't affected by the stomatitis but of course they are now. Chamomile is also a blood thinner so needs to be stopped 2 weeks prior to surgery - probably a good reason not to use it as you never know when an emergency will arise.
He also gets Plaque Off (controls bacteria in the mouth) and I've been trying to give L Lysine but he's not so into it, it tastes salty so I wonder if it stings his ulcers? However, I've seen one person report almost complete control with the combo of those two.
Tried turmeric but Vyktor wouldn't have a bar of it, syringed it in with yoghurt and he just spat it out again.
I looked a cyclosporin but decided the potential side effects had capacity to cause more trouble that what we were trying to fix. My vets usual treatment would be metacam - I also consider that too nasty to try unless it becomes a QOL issue at the palliative care stage. Steroids have often been used in the past (and still are by vets that are not up to date on stomatitis treatments) but the view now is that although you are likely to see some immediate improvment to begin with it is never going to last and is likely to make the stomatitis worse in the long run.
If you're rich you could look at interferon treatment - likely to be more effective if the cause of the stomatitis is viral.
We have pretty decent control most of the time - I judge that based on getting sufficient food into him to maintain his weight and also on the faces that he does or doesn't pull when he's eating. I have discovered that certain foods set him off, e.g. Give him a meal with beef or lamb and we're looking at a good 6-8 weeks of trouble. I also note that he very rarely eats by himself now, I have to take his bowl to him to eat and whisper sweet nothings in his ear, also a lot of hand feeding.
I read an article by a UK vet that reported some of his stomatitis patients were cured just by a change in diet to one without additives. The particular food brand he mentioned was only available in the UK so I have tried switching Vyktor to a predominantly raw home made diet. That was going pretty well until he decided he didn't like it anymore
Q10, zinc and Vit E have been recommended by his Chinese medicine/acupuncture vet. They are on my list for further investigation if it becomes necessary. I try to only introduce one thing at a time so I can judge the effectiveness. If you try the Q10 I'd be really interested to know how that goes. There are Chinese herbs that would be better but they are very bitter and Vyktor won't have a bar of them.
Recently I tried Duralactin (anti inflammatory) most cats apparently love the taste so much so that you have to be careful not to leave it where they can get to it - Vyktor apparently does not. I see that you're already giving that one so I assume that is for something else and is apparently not working for the stomatitis.
Very ocassionally I give him a few days of bupe at a low dose to help him over a bad patch but that has become less necessary with the Plaque Off and as I've figured out what foods to avoid for him.
Bottom line is that stomatitis is the nastiest thing I've ever dealt with. There is no one cure for all cats and you will need to experiement to find out what works for you guys. If you find something magical let me know!
I'm not sure if
@Ann & Tess GA is still logging on (RIP baby Tess I still think of you and your mermaids

- so does Vyktor he has been visiting them lately!) she has a civvie, Emma, with stomatitis that they have well controlled using a host of supplements - I recall it included something with mushroom.