Since ATM’s are plentiful, bringing lots of cash isn’t necessary. Let your bank know your travel plans so your account doesn’t get flagged when you start using it in EC. 20’s are the highest denomination normally exchanged and, no, it isn’t necessary to report cash at customs.
A few years back we switched to Capital One because they don’t charge ATM fees. There are several other banks with a similar policy. We’ve not had a local bank account and just use ATM withdrawals any time we need money. Works for us.