Anyway, I was hoping to get a credit card that I could strictly use as an add-on for Coinbase and I would never use it. Does anyone have any suggestions for what cards to apply for?
If you can't get a single credit card, then your credit score is pretty bad. Every time you keep trying to apply for a card and get denied, it's hitting your credit score even more.
And for your situation, adding a credit card isn't going to fix what you're hoping for it to do. It's merely a BACKUP source of funds in case coinbase tries to debit your bank account and it has insufficient funds, it'll get charged to your credit card instead. You still have to wait a week to get your coins since funds come from your bank account, and there's no way to make the credit card as your primary source of payment.