Boric acid is so weak, you can safely ignore it - it is almost not dissociated. Ionic strength is solely due to the citric acid presence - but it is a weak acid as well, so it is not completely dissociated. To be precise you should calculate citric acid dissociation, calculate ionic strength, recalculate dissociation using activity coefficients, calculate ionic strength again and so on. This is iterative process. Luckily solution is diluted so ionic strength is not large. Much smaller than you calculated (you tried OK, you just wrongly assumed everything is 100% dissociated).