there is a distinct you, its all the cells with your dna, and theres no bacteria inside your body. your stomach and intestines are functionally a well controlled part of the outside of your body. bacteria being inside you is an infection and is generally considered bad for your health. by mass the vast majority of your body is your own cells, so implying most of your body is stuff thats “not you” is pretty hyperbolic.

your argument that it would probably be bad to kill these mites is an appeal to nature. just because theyre there doesnt mean they serve a vital function. im sure there are any number of disorders caused by them, but whether there are any caused by them being absent neither of us can say. maybe there is, maybe there isnt. its possible that no one can say at the moment.