Dang, darn and I'm sorry you are getting hit despite the comprehensive approach you've got going on there.
Quercetin and resveratrol were two of the primary anti-histamine full monty supplements in the regimen that come to mind, I didn't see that you had incorporated. It also suggests to increase the fish oil dosage.
Just listing the items from the QSG below along with the suggested dosages.
Primary Antihistamine Supplements
A. 1 to 2 Grams/day Turmeric (Curcumin) with Piperine
B. 1 to 2 Grams/day Quercetin
C. 1 to 2 Grams/day Resveratrol
D. 8 Grams/day vitamin C
Optional As Needed
E. 2 Grams/day Omega-3 Fish Oil (EPA and DHA)
F. 250 mcg/day Selenium
G. 1000 mg/day N-Acetyl Cysteine (NAC)
H. 5 to 10 mg/day Melatonin (Taken at bedtime)
I. 200 to 500 mg/day CoQ10
J. 50 mg Zinc Picolinate*
K. Diamine Oxidase (DO)** 4 mg 2 to 3 times/day with meals
Have you considered putting yourself into nutritional ketosis? In terms of a dietary measure that has some reports in chronic CH albeit was a small cohort 11 of 15 got pain free, the regimen quick start guide recommends it.
I take it you have the quick start guide handy, if not it is here. There may be some relevant information there (page 12 onwards describes falling from remission / non-response to regimen).
Other reports of nutraceuticals include thiamine which I had seen a few reports recently on Reddit threads of people reporting success but only a single case report in literature using 750mg (tapered).
There is busting... a topic well covered on the forums, if an option for you?
You probably already know you've got some medication options but just putting it out there, perhaps a short steroid course would be enough to disrupt the cycle and for it not to return after the taper? This was my go to for the first couple of years with the regimen (having not stayed on maintenance thereafter all year round). Or another poster pointed out a monoclonal antibody, there's also verapamil etc.
Nothing it seems works for everyone but while you are throwing everything but the kitchen sink at it already I could only but list the options...
I've only been in your position once before and at a baseline 100ng/mL and getting hit with attacks. I did another 600,000iu loading dose, all of the full monty supplements (less the melatonin), did NOT do keto and found myself pain free 4 days later - I realize I am lucky. I finished the load and dropped to 20,000iu maintenance for remainder cycle, level increased to 180ng/mL, calcium was okay - understandably becomes a bit nerve racking at these levels. Pete's labs in the guide show he has been higher than this previously - obviously none of us can flat out recommend you do that.
Good luck I am sorry again you find yourself in that spot. XXX isn't too frequent here these days, if you DM me I am happy to share his email address should you want to drop him a line - hint, it's also listed on the bottom of the full reference guide.