This probably won’t help you but I’m saying it anyway just in case it does help.

I dealt with something similar for over a year that just wouldn’t get better even after I tried all the dandruff shampoos I could find at the store. My doctor prescribed this lotion called taro-mometasone (I think?) and within a week the dandruff was gone and it never came back. This was about 7 years ago, and I haven’t used the lotion since. And it was awful dandruff. It was becoming like scabs on my head and my hair was starting to fall out.