I roughly follow this with some slight deviations (I don’t grind): https://www.reddit.com/r/treeofsavior/comments/45xa7j/leveling_guide_for_ktos/
Up until level 6 cards I usually used my cards until I hit a EXP reset.
At 160 I’ve saved like 122 lv 6 cards, 160 lv 7 cards and 40 lv 8 cards (still collecting). Once I hit enough lv 8 cards to boost me up to 175 I will be using them all.