From what you are describing, I wouldn’t even worry about the backer, and if you do need it, do not use the round rod. We like to use Grip Strip which is trapazoid shaped. Being inside you are not going to get the movement that you would outside since the temp is pretty constant. Being only 3/4” inch thick is not going to leave much room for backer anyway. Don’t get cheap now. Spend the mony for a bucket or two of Perma Chink.