How Often to Change Rep Range?

hey Christian , im planning on doing a 6 week training cycle and within the 6 weeks are 3 x 2 week micro blocks .Each two weeks has its own rep range week 1-2 15 reps, week 3-4 10 reps, week 5-6 5 reps . my reasoning is I normal train between 5-8 reps but feel my back and shoulders are lagging so am wondering if they maybe stimulated by higher reps . Or would a straight 3 x 12 adding load as full sets are achieved be better ? thanks .