I've made that trip the last 5 years out of Dallas. Usually the closer you get to your flight date, the fewer flights and seats available and the price goes up. Not 100% the case as other things can effect ticket prices, but its usually the case in my experience. You will also have more seating choices the earlier you book.
Its whatever works best for you. For me it's much easier to send to my thai bank and use scan and pay through my banking app. But I also have a US bank card that refunds all foreign atm fees, so I use that to have cash to carry. You always need backup, so have multiple ways get your money.