Your dose seems fine, but just know that there is little information out there for long term use. The one and only real concern is possible bismuth poisoning. This is very rare and only documented in those taking high doses and at higher risk (elderly or very young). Still, if you develop any neurological symptoms, you should take a second look at your Pepto Bismol. The good news is that it is reversible by just stopping the treatment. Bottom line is to take the lowest dose and frequency that works.
Sure, resistance can occur, but this is not really an antibiotic in the same sense as typical antibiotics. The good thing about it is that it is useful for C. diff prevention and to keep down bacterial overgrowth of "bad actor" bacteria. It also absorbs bacterial toxins.
Jan