Let be honest here(as someone pointed it out), winning new One Piece readers is hard, because the manga is very long and people hesistate to read all or watch these episodes. Lot of my own friend stopped watching it because it was to long. Only two keep reading it until today.
Winning lot of New readers is for One Piece impossible. But I think this shouldn't be also Oda goal. I mean he had over 15mio volume sales in the past, I think he rather should focus on what he and One Piece always had: long term readers/veterans. Focus on them should be more what he should care, make them happy>having a higher sales.
Why you think 2023 sold more(2mio) than 2024(Endphase of Egghead)?
Because even thought we been in the middle of egghead, we got lot of crazy shit with made people buy the volumes:
Cross guild, Greenbull/Shanks, Law vs Teach, start of egghead(middlephase also), Shanks vs Kid, Lucci vs Luffy, Vegapunk plot, Reverse, Imu and Gorosei Monster reveals and Cobra death, Garp vs Kuzan..
Content like this with a good pace is what we old readers want to read. The Endphase of Egghead Was luckluster, stratched and kinda(sadly) boring, people lost interest and fun reading it, a reason why this year it "only" sold 5.2M...
After wano people can't take anymore long arcs(for right reason), this poor dragging of the story need to be handle better.
Believe me, make the story about Shanks and Teach and write it with a good pace and all the old readers will come back and the sales will reach 10mio again..
Oda doesn't need to win lot of New Fans, he already has a huge amount of old readers. He just need to win these readers back and One Piece will reach 10mio sales again.