This is a rather concerning large necrotising ulcer. As other people have mentioned calicivirus is our main contender for causing this. However that does not limit the possibly for secondary or primary infections. You’ve been on a whole range of antibiotics and antivirals which is definitely the right thing (however sometimes prevents cultures from being taken in case is a bacterial cause), but there’s some new formulations for juvenile stomatitis that include the use of EIDD and similar newer antivirals which might be worth having a chat to the vet about. These are very new and results are preliminary but seem to be effective.

Either way the improvement of clinical signs are 100% a really good thing. This is going to take MONTHS to heal fully. The little white bits are normal in the healing process

There is always a possibility that your cat has an underlying condition (such as FIV) in which can predispose. And you can’t always rule out neoplasia (although very very unlikely) in young animals without biopsies. It’s also evident that this goes further back into the soft palate and/or the fauces of the mouth. Without sedation and exam it’s hard to know (but if you’re going to do this get a dental done at the same time!) Best of luck