I do the same thing; large dry sac on rear rack with smaller dry sacs inside. I actually keep the wet tent inside a dry sac, then wrap my sleeping bag around that dry sac, synch the whole thing with wedding, then squeeze it into the larger dry sac.

Many dry sac lines have a "durable" version. I would recommend that for the outside sack. It resists friction better, and there always seems to be a little friction somewhere.