I don't think that would have made things better. Hoshido would still fight and lose and casualities would still happen(arguably moreso than originally because they lost their 4 strongholds) because Hans and Iago are asshats who will find new ways to make you suffer. Sure the war might not last as long but the amount of lives lost would still be the same if not more. Also if all 4 would have died right there, I am pretty sure ghost Takumi/or any other sibling wouldn't have given their blessing to you to repair the yato since at that point they would definietly hate you for what you have done.
Which means zombie Takumi(who can still be possesed even when dead) would have killed all the Nohrians and increased the death count.
Generally speaking, chapter 18 conquest felt pointless to me and should have never happened. That chapter could have easily lead to a different kind of conclusion if Corrin actually talked with his Hoshidian siblings about the war. This chapter alone had so much potential yet they threw it all away, never mentioning it again.
THIS. While Corrin did some really stupid things he/she is not alone to blame since you can excuse it with lack of experience. If the Nohr sibling weren't so stubborn(especially Xander, dear lord what a misguided tool) Corrin wouldn't be forced to use such a flawed plan.