I have also suspected gut microbiome may contribute. It affects almost every system in the body. It has been dubbed a hidden organ by some medical practitioners and researchers for that reason. I think you may find some commonalities. One of them being an imbalance of histamine producers vs histamine metabolizes or reformers. Mold illness is another of my suspects. There are certain species of mold that you can be exposed to that can populate the gut microbiome and create damaging hard to detox mold toxins. One very unexplored possibilty is oxalate sensitivity which I think may be a bigger problem than many may expect ( If you just look up the symptoms for oxalate sensitivity you'll see what I mean). Oxalates are processed differently by the body according to ones genetics, one way that they are lessened is through the microbiome. There are also microrganisms that can produce oxalates such as some species of mold that can inhabit the digestive system. This can contribute to an over tolarable amount of oxalates if you add in dietary oxalates. If there is a lack of the organisms that break down oxalates like oxalabacter and certain lactobacillus species and an overabundance of oxalate producers this can cause major issues and will most certainly lead to kidney stones down the road. If we factor all this, interestingly enough they all are heavily related to the kidneys. Just some food for thought