Why not give Fervor a BONUS AD ratio?
Lower the base damage, hell, just remove it and add a bonus AD ratio. Nothing super huge, maybe something like 0.065% of Bonus AD per stack. Most AD champions have a base AD of around 90 - 110 by level 18. and most adcs have around 200 - 300 bonus AD by then. If you have 200 BONUS AD, Fervor will deal 104 damage per auto attack at MAX stacks. if you have 300, then it will deal 152. Maybe instead of 0.065 it could be slightly lower, like 0.06. But you understand. At level 18 the base damage for Fervor is around 111. It's also possible to have an even lower ratio but halven the base damages.
The purpose for this is to prevent champions that are taking fervor and building tank whilst doing lots of damage (Tanksuo/Tank Ekko) from abusing Fervor.