First, you probably don't have dandruff---just a really dry scalp, which the dandruff shampoo may be making worse.
I have never used Wen, but I had my thyroid removed due to cancer last year. I have found since then, that I don't need to wash my hair every day. Like you I have had a lot of hair loss, but I was told to expect it. I used to have hair so thick I could barely use a pony-tail holder. Now I can go around 3-4 times. My hair still looks healthy, it is just so much thinner. Honestly, my hair was so thick before I had never seen my scalp.
To keep the flakes to a minimum, I just wash it every other day. Also I drink an obscene amount of water. I found that not only was my scalp dry, but so was all of my skin. This has helped tremendously.
Talked to the endocrine about all of this, and she said it is to be expected. She had some ideas to treat the "symptoms", but basically said stay hydrated, lots of lotion, and use condition on your scalp. I told her I stopped shampooing daily, and she told me to let her know how it went--so she could pass on the tip to others if it worked. Since she has a functioning thyroid she can't really experiment on herself.