I think you have to make
every card have a (minor) damage increase—or
no card have one—plus some more-powerful alternate effect which is more varied (and where which is "best" is situational).
As purely hypothetical examples (these would almost certainly not be remotely balanced; I'm throwing them up here off-the-cuff between work meetings):
- Arrow: Gives a +2% damage increase plus an +8% SpS/SkS buff to target for the duration of the card.
- Spear: Gives a +2% damage increase plus an +8% Crit buff to target for the duration of the card.
- Ewer: Gives a +2% damage increase plus halves the MP cost of every cast target makes for the duration of the card.
- Bole: Gives a +2% damage increase plus extends the duration of all timed beneficial status effects on the target (or applied while the card is active) by 20%.
- Spire: Gives a +2% damage increase plus decreases the duration of all timed detrimental status effects on the target (or applied while the card is active) by 20%.
- Balance: 'Locks' a card on the target, allowing a second card to be played on them without overwriting the first.
In this hypothetical, everything (except Balance) has a very small damage boost (so no one card 'wins' by being 'the one with the damage boost'), and there's still variety in what could be played. Bonus: time manipulation shenanigans with Bole and Spire.