Specifically regarding on hit effects:
My interpretation of why Larian may find it difficult to have on hit effects as a pop up rather than picking them in advance, may be partly to do with animations. E,g. if an attack that uses divine smite is intended to have a glowy weapon effect, they can't have you swing a non glowy weapon - hit.. and then retroactively add the glowyness.. which I think is part of what the guy i nthe video was talking about with menacing attack "you can hit and then decide oh it was a menacing attack all along, and it works fine in tabletop because its all in your head"
Like the OP suggested, you can have 2 version of divine smite, 1) activate on hit, 2) activate on smite, and choose which one you want to trigger. And I dont think the animation part is hard to implement, you don't need to redo the animation because in a PC game, dicing is done automatically by the computer, and it is calculated before the real animation happens because miss and hit shall use different animation. So put a trigger between the dicing and animation is totally doable, so if you roll a nature 20 and triggered the smite trigger u setup, the computer will decide to use the smite animation. I don't see any difficulty here.
But at the same time, pc game and trpg is different, you do feel very excited when u roll a 20 and decide to use smite on paper, but if u put it on a trigger and it occurs randomly, i'm not sure if it will be as exciting as it is on paper.
