Solasta went for the literal implementation - after you land a hit you get a menu popup, where you can add smite to the attack and choose which level you want to expend
BG3 is unlikely to go for such interruption - The most sense, I think, considering what we already have would be something like “sneak attack” - an ability, with smite already assigned to it, that we decide to use instead of vanilla attack.. It should also act as spell and have expandable up scaled versions. The spell slot should be consumed, however, only when hit registers.
That would simulate entirety of PnP choice, however.
yeah, I think that's precisely what they'll go for, similar to the battlemaster abilities.
You choose in advance if you use it or not, but as you said the resource/spell slot is consumed only if the hit registers.
Of course, this won't allow the granularity of "deciding to use it only if it's a crit", but honestly it would be a reasonable compromise and not something I'd twist my pants about.
