Where is it implied Mihawk’s bounty is bc of the past in the spoilers? Besides even if that were the case, shanks isn’t ruthlessly attacking evetyone but imagine if he were like Kaido and BM. Bounties would be even more distant
They mention his title from past specifically, so I'm assuming it has something to do with that.
Anyway, targeting Marines does bring its inflation but it's still not as insane as being the sole leader of an absurdly powerful crew, as well as an Emperor's armies, territories and resources amassed over decades.
Mihawk is definitely up there with Yonkos, solidly so after this. Trying to pretend the insane inflation potential of Yonkos doesn't more than make up for 500m is about as delusional as trying to pretend Sanji's actions in Wano don't put him far above Jinbei compared to what bounty entails