We are the Pawg, resistance is futile

Well fuck.

Although I’ve been using it on and off for 30 years so I’m probably fucked already

In the UK. I’ve found the best treatment has been coal based shampoo. I use T-Gel.

Lather hair with normal shampoo to remove hair gel/dirt/sweat and then rinse and repeat with a small amount of coal based.

Leave it to soak into my scalp for a few minutes before washing off.

3 in 1 shampoo is terrible for yoir scalp.

Tbh it doesnt sound like you have dandruff, more like psoriasis or plain dry scalp.