I applied using income docs as my company was on the approved list that they have. No idea if they accept C2C or not. You can check in the SC thread or contact the branch and apply offline as their online application has some issues.
I think DCB gives 3x on instant vouchers so you can do the math. Plus it will help hit milestones on DCB as well.
Moreover, you can buy instant vouchers for other sites as well like Swiggy, BB, etc which don't have any extra charges for site-specific usages.
I think Eterna for online spending until its sweet spot each month and SC Ultimate should be the go-to option. For offline spending, Eterna's reward rate isn't good so SC Ultimate can be used. Moreover, SC Ultimate gives 1 reward point for every Rs 30/- so helps if the amount is small to collect...
I will close Ultimate once I receive DCB. A combo of Eterna and DCB will be fine for me. First-year they give 6000 reward points so only taking a small hit.