Unfortunately, the 5* guaranteed banners can only be pulled via purchased Star Quartz, so the free ones earned in-game can't be used on them. Your best best is to wait until there's a nice 4/5* Servant, as well as CE, that all have a rate-up at the same time and try your luck then. I've heard the CE during new Servants releases continually get better, so at least later if you fail to pull a Servant (and let's be honest, you probably will), you'll hopefully get a useful CE or two out of it.
Basically, pull for characters that interest you but don't expect to actually receive them (unless you open that wallet). It's harsh, but that's just the way this game is. Thankfully, they're not NEEDED to actually complete the game, but it doesn't make me want characters such as Jack, Tamamo, Mordred, Jeanne Alter, etc. any less. My plan is to save up for the characters I really want, try to pull them, and if I don't manage it, just pull on the guaranteed gacha every time they're around and slowly build up my roster. I don't expect to be like many of the Japanese players with 25+ 5* characters, my luck just ain't that good.