@vthailey
About diet, I've stopped eating gluten almost entirely and have had success. This is just my experience, but hopefully it helps.
I had HHD flare ups for a couple years. Flare ups happened a few times a year across my chest, neck and forearms, plus constantly under my armpits and on my torso. Not the scalp like you mentioned.
Dermatologist biopsied my armpit to diagnose the HHD.
A family member told me they had gone gluten free and it helped, so that's where I got the idea.
I stopped eating wheat completely and I've entirely cleared up. I haven't had to be as extreme as avoiding cross contamination. Then eventually I could eat a few bites of things containing wheat flour occasionally and been okay. I mean a bite or two every couple weeks. That's been a year and a half ago. There are so many options to go gluten free it's very doable once you make some adjustments and have some support.
There are a few other people on this group who say they do low carb diet, low sugar, reduced tomatoes, etc.
I asked my dermatologist about going gluten free and he said he didn't know that it was studied. But it's what has helped me.
About other things I do to manage it: I shower soon after exercise.
I use a body powder in my groin to keep it dry. Air out after showering. Manage stress, my first big flare up happened related to a very stressful situation, so trying to be aware of that. No idea if anyone else saw a connection to stress. I don't see that on this group.
If I overdo eating some gluten I get a small sores first under my armpits, so I use mupirocin to prevent it getting infected and then get really diligent again.
As I said, just my experience. Hopefully you find relief.