So having 2 Medica II's and 2 Regens on the whole party isn't OP, but stacking astro card buffs is?

Sure, it may be OP for trash and overland mobs, but trials and bosses?

If you're doing .01% damage to a boss, at best with 2 +150% Balances, the damage buff is +30%.

And what is +30% to .01%? .013%. Which is insignificant in the grand scheme of things, not to mention its only for 1 party member.

Besides, even if running content as full party of 8 astros, the total buff to everyone in an optimum situation if Balance cards could stack is +40%, which increases everyone's damage to .014%.

Which again, is still insignificant overall.

Astro cards have never been game breaking or required in any content. They're just there to smoothen the ride to the end.