For GI-based mast cell issues, oral Sodium Cromoglycate (aka “Nalcrom”) in capsule or liquid form seems to be the best thing. The dose I take is 600mg per day (6 capsules) and I’ve seen other people on 8 per day. For me it helped a lot with nausea, diarrhoea, bloating, and reflux.
The main drawback in my case is that it’s fairly expensive for me but that will depend on your insurance situation! Everything I’ve read seems to indicate it’s a quite safe medication to take.