What if conqueror worked more similarly to old fervor:
Dealing damage to enemy champions grants a stacking bonus AD for up to 8 seconds (max 5 stacks, max 10-60 AD, based on level). Upon reaching maximum stacks, convert 5-10% of damage dealt to champions (based on level) to true damage for 4 seconds. Melee Only: while at max stacks, damaging an enemy champion refreshes the 4 second duration. Cooldown: 6 seconds.
Or something like that? It's similar to what you have, but the pattern would feel a lot better in my opinion.