If you want ADCs to stop coming online at 2 items
Why not just make their attack range a scaling stat? That way their early game is significantly weaker, but they still scale well with levels and gold. This would also make mages and melees more viable bot because the reduced range reduces their ability to harass from safety. Just say nerf all ADCs starting range by 100, and make them all end at like 25 higher than what they are right now since in a duo lane they are unlikely to even reach level 18 unless the game is 50 minutes. So Ashe for example starts at 500 and ends at 625, Sivir starts at 400 and ends at 525, Cait at 550 to 675, etc.