Agree that after final war there will be no bounty increases (probably straight frozen).
And it's true that the more you go up the harder it is to get increases but the finding of Laugh Tale and the One Piece (the drop that thestroys the dam) won't be reduced to a 1.000.000.000 if we got a buff twice (or x3) as big.
Including post Wano there will be a total of 3 more buffs imo, which won't be possible if Luffy goes to 4.000.000.000 already.