You might be replying to the wrong person, I've never mentioned threat level, my whole point is I believe bounty for most people, is a gradual increase, not necessarily assigned exactly to the things they did last.
Defeating a pirate with high bounty, would lead to a re-evaluation of your combat skill, which may or may not result in receive a higher bounty than the person you just defeated.
Pica is already a competent enough pirate to receive 500 mil bounty or more, just not when compared to Luffy, Law and Zoro's bounty. People need to stop using Luffy, Law and Zoro as scales, they are way stronger than their bounty suggests, it's like using Sabo to measure Yonko commanders.
Pica has def shown enough competence to be worth more than 300 mil, you have wayyyy less competent pirates that has bounties of 300 mil or more, such as Caesar, Pekoms, Pedro, then Tamago in the 400 mil range, Izo in the 500 mil range, like if Caribou and Bellamy can get 200 mil bounty roughly, 300 mil is NOTHING to Pica.
I don't see why due to bounties being more difficult to increase after 300 mil, would make bounties below that range irrelevant... They are still bounties.
Zoro beating Pica, getting 320 mil, does not mean Pica must be 320 mil or less, it does not work like that, just like Luffy defeats Croc, but many arcs later, when captured, Luffy gets sent into ID level 5, while Croc was considered level 6.