There might be another way to go, as well.
Instead of tightening the focus on the nostrils in order to set aside the other distracting input, you might try blending all the input together such that, no matter what else can be detected about the body, the breathing is noted as a repetitive body-flexion encompassing all the other bodily inputs. This, then, can be the basis for the third step of anapanasati's first tetrad.
"Others will misapprehend according to their individual views, hold on to them tenaciously and not easily discard them; we shall not misapprehend according to individual views nor hold on to them tenaciously, but shall discard them with ease — thus effacement can be done."- MN 8 - Sallekha Sutta