Bounty gaps before a raise are meaningless. Kid and Law were a billion behind Luffy at the start of Wano, end of Wano all three were level.
When Usopp had a big role on Dressrosa, he went up past Sanji
When Sanji had a big role in WCI, he went up past Zoro
Usopp has been set up to have a huge role on Elbaf. Oda has absolutely not delivered on that so far, I agree.
If Oda does deliver though, I think it’s quite clear based on how Usopp was treated in the past that
-he’ll get a big heroic moment with a good sniping feat
-that moment will make him a hero to Elbaf
-he will lead in battle the warriors of Elbaf, strongest army in the (remember, it was Hajrudin, future King of Elbaf who held Usopp up as God Usopp originally)
-he’ll be established as a top three for Luffy like his dad is for Shanks
That’s easily enough to put him in spot two or three bounty wise after his big arc. The post Elbaf bounties are likely going to be Luffy ~ 4 billion, top commanders of Zoro, Sanji and Usopp ~2 billion, lot below that generic boosts of about 300 to 500 million.
Oda likes his bounty gags. Sanji’s furious about something, Usopp gets scared of his, Chopper’s is shit, Franky’s got a picture of someone else. Elbaf is prime Usopp gets beaten up by Sanji for being higher than him territory.