Rosy boas do not come from sandy areas in the wild. They do come from dry climates, but its not like the Sahara or anything
Sand will only lead to problems of impaction of the organs and in the nose and mouth, which can lead to mouthrot as well. They also will have scratched scales and eyecaps. Stick with the aspen. Other alternatives are newspaper, paper towel, beta chip, Carefresh, and crushed walnut.
As for the bark. I wouldm't recommend going with any fir bark just due to the fact that it is a softwood. All softwoods produce oils that are mildly to severaly toxic to herps and small animals. Pine being a very mild one, more of just a mild respiratory irritant, cedar being a powerful neurotoxin. Fir bark is very mild, however I still wouldn't. It isn't so much the humidity, as with bark you can choose your humidity levels by how moist you keep the bark.