I am not a dermatologist. Malassezia yeast thrives in warm, moist environments. If you don't already dry your scalp immediately, definitely don't leave a towel on top of your head for a long time. Realistically, there is not a "cure" yet, but longterm management.
It often improves at the beach due to UV light which is antifungal and anti-inflammatory (in this context) and saltwater does have some antimicrobial properties and helps break up the scale. Saltwater also has other minerals compared to store salt (depending on the kind). De-stressing also helps. The condition often flares up during periods of stress. They do use UV therapy as a treatment option (usually for more extensive involvement not just scalp).
For a medicated shampoo (antifungal) to work, it is best to first loosen the scales with coconut (not olive) oil, salicylic acid, or tar shampoo. Dermatologists can prescribe a higher strength shampoo. Natural remedies that seem to have the most "proof" are coconut oil, tea tree shampoo, apple cider vinegar, diluted raw honey, aloe vera gel, antifungal essential oils like rosemary.
Not sure if getting a difficult case checked out (once) professionally would be worthwhile. I do understand the reluctance, especially with needing to renew prescription over and over.