The best way to use DCB is to not redeem reward points in cashback but to redeem it for flight and hotel booking.
The greatness of DCB and Infinia relies upon their redemption against hotels and flights. Even if you are not a frequent traveler, domestic flights these days easily cross 5000Rs.
Since the reward points are valid for 3 years for DCB, there is a high chance that your points will be redeemed before expiring.
Against Millenia, overall reward is lesser than DCB but does not bind you to hotels and flights that one trade off that I see.
However, what I like about DCB is also the monthly and annual milestone rewards (If you can somehow unlock it)
Also, SBI Cashback is a direct competitor with Millenia and in many ways better. So even if you don't have Millenia, you can still get those same benefits
So upgrading to DCB and then applying for SBI cashback seems to be best of both worlds 🙂
Good luck.