Auto-crit actually anti-optimzies with crit fishing builds.
If you expended resources to roll 3d20 pick best and 19-20 crit range, you already have a 27% chance to crit per swing, and almost 50% per 2 attacks.
It optimizes with things to deliver on a crit, but against the rest of the package.
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Note that all smites are melee range; so concentration is a great mechanic, as can be lost easily. It reduces the benefit of an auto-crit if you can lose it (or have to invest in not losing it).
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Suppose you are fighting 5 monsters with 5 players, and the combat lasts 10 rounds.
Each round there is a 50/50 chance a monster drops. So you either have to not focus fire with your ability (or the party cannot), or you risk losing it regularly.
If everyone used it, and it did no extra damage, you'd be fighting an extra 0.5 to 1.0 monsters per round as you attempted to avoid "blowthrough" and delayed damage. That is 20% to 40% more damage over the entire fight (really; 1+1+2+2+3+3+4+4+5+5 over 10 is 2.5, adding 0.5 to 1.0 monsters for every round is gonna hurt).
Against a single big monster that lasts ~5 rounds, you just have to know not to use it towards the end of the fight.
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My point is you want it to be a tough question to use it or not. And I think it should be as useful as other cantrips.
Take Toll the Dead and Fire Bolt. Having Toll gives you a 18% damage boost on wounded targets, having FB gives you a 22% damage boost on unhurt targets. Most people wouldn't take both cantrips; they'd pick one of them (usually Toll). If 1/4 attacks are on full health targets, toll adds 9% damage over FB. Taking both is another +4% damage (and opening with FB, then using Toll later).
That isn't generally considered worth it.
True Strike (with crit and advantage) + FB on 60% hit chance is 5.5*.65*2=7.15 vs 11*.81=8.91, or 25% damage increase.
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Another approach. True Strike grants advantage, and if the lower attack die would hit the attack is upgraded to a critical hit.
This Strike + FB with 60% hit chance is .81 hit 0.4 crit. 5.5 * 1.2 = 6.6, less than just casting FB twice.
"Your first attack on the target has advantage, ignores disadvantage, and ignores 1/4 cover and 3/4 cover. If the lower of the two attack dice hit, the attack becomes a critical hit."
Still probably not good enough. Very niche.
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Or:
"You have advantage, and the attack deals 2 extra damage dice from the weapon or spell. At level 5 you deal 3 extra damage dice, 11 4, and 17 5."
This is an attempt to have it scale in a crit-like manner, but not actually be a crit. So smite-fishing doesn't work.