One thing with seborrheic dermatitis is it is affected by diet in a big way and often shampoo wont really help that much,food like gluten/wheat can cause a flare up even if your fine with this food in other ways as its an inflammatory food as is citrus fruits too,so try cutting it out for a few weeks to see if you notice any change in the condition usually it will calm down,
other things to help healing is vitamin B6 this really stops symptom flare ups in days i used to take it for the same thing,i used take around 35mg a day but you can go up to 50mg a day this is not a cure but will control and bring relief of it,mine stopped itching in 2 days of taking them,
but i have now gone gluten free and it is totally gone there is not a trace on me,like you i tried the shampoos you mentioned you could say i tried all that is available but it worked a few washes then not afterwards,this is because it was from within so shampoos were a waste of time as it only brought relief for days then nothing,
you could try writing a food diary to see when the flare ups are worse then check to see what you have eaten 48 hours before boring i know but worth putting the effort in,i did this after a tip off someone and came to see that gluten and citrus were the cause of flare ups,some people get it with dairy foods too so keep the food diary going for a week or so to see when your worst,
gluten is in drinks and everything so you need to be strict about the diet or you wont see a difference,if i eat it by mistake a get a big flare up in 2 days without fail,that's why checking labels is vital to be successful,also cod liver oil is excellent for this as its a natural ant-inflammatory so helps heal the skin from within,you need patience though as cutting gluten can take around 4 weeks or more to show results as it takes that long to leave the body,
it is a well known fact gluten and wheat make seborrheic dermatitis worse or even be the actual cause too,one thing if you use hair dryer then don't this makes the scales more scaly so making it worse and dont let the water you wash it in be too hot again this flares it up,
not sure if i have been of any help but i had it for 17 years and tried hundreds of shampoos and was seen by many doctors and even a dermatoligist but all they did was send me away with endless creams,lotions and shampoos all of which did not help,but i discovered the gluten making it worse by checking things on line and had tips off a friend then it clicked in my mind and i have not looked back i am cured!
hope this works for you and also this goes worse in winter as its seasonal not sure if you have noticed that detail yet,best of luck and i wish you get better from it soon.
My experience of seborrheic dermatitis of scalp and eyebrows for 17 years,now cleared.