You’re very unlikely to find specific legendaries in packs unless you already own them. I’d stick with what you can craft and pick a class or two to stick with. It’s not as fun as building everything you want but it’s kinda mandatory for F2P until you’ve been around a while.
Here’s another post I made about crafting guidelines. The thread is about being economical when buying cards with money but that just means it applies even more for F2P.