iOS third-party keyboard with functionality to swipe down on letters to enter numbers/symbols?
FancyKey by Pinssible Labs allows for this, and is the best replacement keyboard for iOS I've used. Setup is simple, and turning on Swype is as easy as switching to the FancyKey Keyboard, tapping the gear icon, and tapping the "Swipe input" switch on. Your keyboard with default to the custom keyboard until you change it by tapping the globe icon.