It's actually an issue of everything being buffed directly because of power creep. To mediate it the solution is to slowly revert changes back to previous stats such as damage reductions for weapons to when they were introduced etc. As for what I am doing right now, to make guns suited for their tiers; I reduce the guns considered OP to bring them in line with guns which are outshined by others, rather than buffing the weaker guns. Along with this I'm reducing the stats for each gun level. I prioritise things such as reload for bow weapons, damage for assault rifles, accuracy for sniper rifles etc. Each level will scale differently by rarity, and the only reason it takes a while is because I'm lazy and don't want to dedicate 24 hours to zs a day. I aim to bring things back to a level where zombies take 4 hits to kill a human with 100hp, rather than 3. This means that their damage to humans shouldn't go above 33.3.
Despite that, I think the card system is fine, it's just been managed extremely poorly by ignoring the consequences of stacking buffs against nerfs and vice versa; so yes, rather than adding new content, I'm prioritising reverting and mediating the issues which have come up as a result of said balance issues.