You may want to upgrade your seat. Ride a friends bike to see how their seat feels. I rode 5 seats before finding one I like. Don't upgrade wheels yet... if at all. Since it's a Giant I know what you bought it from a shop... which means if the shifters and derailleurs are not working well you can take it in for a free tune up. Do that first before you upgrade parts. I dunno about your duck feet. When you want to rest it probably doesn't matter if you coast or raise your cadence. Probably better to keep your legs moving though. I know when I top a climb and decend the other side I make sure I still pedal when I can or else when I get to the bottom I am too tight.

Good luck!