I tend to keep GF cards until later to see if I can max them out. I guess there's no rush to burn them, you can always do it later once you have a clearer image whether they'll reach the level you want or not.

Maybe it's good that you don't play survival. I play it up to a few times each season, but for example now at the beginning of the season I forgot to disable my main deck on survival and all or most of of it is now in cooldown. I lost a lot from that mistake, because I can't play Ranked at least for another day when my legendary cards and GF epics get unlocked. And even then, I'll probably lose more than win until the rest of the play deck gets back from cooldown.