or if you intend to roll only attack, make it scalable of attack roll vs Armor.
have fixed damage,
unarmed 1, dagger 3, greatsword 8, or what not.
sneak attack +2 per rogue level, as you said, smite +5 per current d8.
then when you hit you deal said damage, but;
beat AC by 5, +50% damage
beat...