In case this helps anybody -
I was diagnosed with MCAS last December (2020). At that point I literally couldn't tolerate a single food, medication or supplement without reacting and I was very under weight and malnourished.
6 months later I am still in the early days of recovery but one thing that has definitely done something positive is VITAMIN D. It was the first supplement my doctor (an immunologist and MCAS specialist) advised me to take and while its only gotten me from a 2/10 to say a 3 or 4/10 it's at least enabled me to tolerate medications.
Right now he's got me on 2 antihistamines (loratadine and famotidine); these haven't yet had much effect and presumably he'll be trying some other form of medication when I see him next month.