Speed up the marksman power curve to match mid laners, then weaken their late game to compensate
Imo the worst feeling is a game ending before I can have any influence
ADC having to wait till late game only works if there is an actual late game and they have to be really strong late for it to be worth the wait.
Just let them power spike around 6 like everyone else, that way they can function better outside of bot, won't have games ending before they can feel useful and won't have to be overtuned late