What should I do with all my extra cards???

you can build a backup deck like you said, or trade them with mga kaibigan who have cards. and of course if there are some cards left i suggest you keep them in case something happened, they don't take that much space. like me for example, i used to have between 150 to 200 cards and Nawawala all of them cause i moved. well, here's my opinion:)

I constantly find old cards I hated, then realized they were useful in an obscure way that didn't become clear until now, but as a whole they don't do you alot of good. I pag-ibig mine for nostalgia, but if you honestly dont care much for them, here's the three things I do.
Sell them to Trollandtoad.com, they buy any of them. make a lil cash and make some other kid very happy down the road.
Make them a present to a sibling or friend who plays (or wants to play). they might have use for cards your don't, and it's like a super sized booster pack for them to look through.
And lastly I use all my useless ones to bulk up sleeves for cards I printed from an online card maker I use. gives the sleeve the same thickness for use with normal cards in a deck.
Hope this helps, this is just the things I do :)