The main goal is to keep a healthy scalp.
Have you tried Head&Shoulders? Their shampoos are really good. I can also suggest Neutrogena T Gel shampoo. I used to have dandruff frequently in the past and what seemed to work for me is using a neutral shampoo to wash all my hair and scalp (if your ends are dry you can always use conditioner after), or a specific shampoo for irritated scalp (do you know the brand René Furterer? It has one that is really good), but it means the use of a separate one for long hair, or at least the use of conditioner because it's not so adapted to the lengths. I don't know if it would work for you, but Kerastase has a shampoo called Dermocalm which does exactly what its name suggests, but it's on the expensive side.
What also seems to work for me is i tend to have a dry scalp (and considering i have combination hair, that's quite ironic), so what i do is about once a week i do an oil mask (i use monoï, which is coconut oil with tiare flowers macerated inside, coconut is fine alone, or you can use camellia oil), and to that mask i add a few drops of panthenol, aka vitamin B5. It helps preventing and fighting against dehydration, so that i cover both sides. I usually leave it on for the night and then wash my hair with a neutral shampoo.