@Runner5 My MCAS had been so well controlled for a few years I didn't realize I had it til my ME/CFS doc suggested I had it.
The reasons? I'd figured out what foods that set it off and avoided them, and the naturopathic doctor I'd been working with over that time had be on significant doses of vitamins B5, B12, C, folate, and curcumin which each hsve a role in calming symptoms down.
I started IVIG 5 months ago and had severe intestinal pain. which turned out to be MCAS, with prostaglandin D2 and chromagranin A being high on lab tests.
We added Benadryl, fexofenadine, Ketotifen, ondansetron, and boswellia, and raised my B5 dose which managed the symptoms during and after IViG. The next step would be an H2 antihistamine, which it looks like you're on.
As for going off supplements to see if it helps, I can tell you I've had Farr more problems with prescription drugs, most of which contain corn or milk derivatives, which I react to, causing me to need them compounded if they can be made at all, or in the case if Benadryl, I can only use the injectible form.
I take a lot of Thorne Research supplements which are hypoallergenic and carefully read labels on everything else and dig into how they were derived - most vitamin C is corn-derived ascorbic acid, so I find fruit based vitamin C, and I broke out in hives from B12 shots grown in a corn medium, but was able to find a different B12 not grown in a corn medium.
Best of luck in sorting this out!