Do I need a backer rod?
You should stuff backer rod in where the depth exceeds 1/2", to conserve your sealant. That self-leveling sealant will flow/drop to the bottom of the opening rather than bridge the gap. You would get around 12' of fill from a single tube with a bead exactly 1/2" wide by 1/2" deep, much less than that if you don't fill it.