I also have these two cards. I use CRED mostly, rewards are not fixed. Mobikwik gives 2% cashpoints that has expiry then you can try GPay, Amazon Pay but rewards are not guaranteed.
If you have HDFC Easyshop DC, Pay CC bill thru Airtel payments bank on Paytm/Amazon (avoid Airtel Payments Bank... merchant name)
For HDFC Millennia DC, you can pay CC bill directly on Mobikwik