what youre asking for would probably be a nerf.
turning the ad scaling to total ad scaling and reducing it so it ends up at the same value as now would mean as soon as he gets that massive ad steroid from his passive it would end up dealing less damage than it does now.
giving him the full ult reset at lvl 11 makes more sense if you want to buff him.
it wouldnt be that much of a buff, since the current 20second time window for another cast after an execute is already decently large on the ability, but it would be somewhat decent compensation for the most recent nerf they hit him with when they fixed the bug with his q.