You can't change squalor but you could add health / happiness / law bonuses to the higher city level government buildings which tend to counter the effect of squalor.

Unfortunately you can't adjust the population v. city level relationship so highest is 24000 people for huge city level.

Everything after the huge city level would have same government building , you could add a further building including bonuses with long build time that can be started at huge city level but I can't think of more specific way to relate bonuses to population level over 24000.