Axis cashback's return is way below 7%. Assuming you max it out , you get just (0.02*5000 + 0.05*35000 + 0.07*30700)*100/70000 = 5.7%.
Max you can spend is 70,700rs but with SBI CB you can go upto 1L.
Till 40K per month your effective CB is (0.02*5000 + 0.05*35000)*100/40000 = 4.6%.
Only when you hit the sweetspot of 70,700 it rewards you 5.7%