Hi! I had severe dandruff from my early teens until about 6 months ago. I would wake up every night needing to shower because my scalp would be so itchy. The 2 main things that helped me was to stop using shampoo all together (No-Poo) and start washing my hair with cold water instead of warm. Ever since I implemented those things to my shower routine my dandruff problems seems to have vanished. Have you tried experimenting with your diet? I myself eat mostly meat and have heard numerous people who have had success on an animal based diet
I use a specific oil for treating the spots (blackcurrant oil sold for head/face use) as soon as I feel it popping up where it is more like exzema rather than dandruff. It can look a bit greasy so I try to not overdo it but keep it in for a few days, reapplying if washing the hair. More oils can likely give similar results as long as they are scalp friendly.
For dandruff: like someone already mentioned, transit to a non tensids schampoo or just hair balm without silicone or other drying ingredients. I have a specific one to prevent itching that I use just on the scalp and balm for the rest.
Then brush it with one of those brushes (not sure what it is called in english, barely know in my own 1st language) that is made of natural hair-ish very tight, used to make the hair shine. It will balance the fat in the hair since most of it will be up top (the hair wants a lil bit of fat), and also gently remove a lot of the dandruff while massaging your scalp.
The worst thing you can do pretty much is use dandruff schampoo that really dries out the scalp. It can help for a bit but will cause more problems long term, until it is hard to keep up washing it before it´s yucky. With “my” method (took a few months to transfer into admittedly), I can wash it 1-2 times per week without it getting yucky, if also doing the rest.