MC's plan isn't that bad, if he really thought the monsters would evade the wall + him killing a few to drive them away and the village would be left alone. He's a battle hardened soldier, so I guess he knows how monster stampedes work in his world.
But then the whole plot of the "save the village" arc rests on the fact Yet, it just dawns on him, that this is unnatural. (He also notices this, because he has that expert knowledge.) But at the same time he can't be possible that well versed in the field of stampedes, because he overlooked the glaring fact, that so many different kind of monsters showed up at his village back then. All at the same time! He might not have had a overview of the numbers, because it was too chaotic back then, but he saw the dragon and a few other type of monsters. He had everything together to add it up and notice it earlier. Much earlier in fact.
It's as many already pointed out. He had over 10 years in his second life, to figure it out. But even before that, he fought years against monsters in his old life. That soldier life, in which he supposedly learned enough about monster swarm behavior, so that he "knows" how stampedes work. Shouldn't he have already figured out in all those years, that that could not have been a natural occurrence?
Oh man. I seriously hope that old man doesn't blow a hole into the wall. And in case he does, that he can come up with a way better explanation than, "Because I wanted to get to my baby boy." If he does and he can't (or gives another stupid explanation, that doesn't include the absolute necessity to open a hole) I .... I don't know, but in the last few months updates of good fantasy (including isekais) became less and less, some even turned bad and I did actually not feel like picking some of them up again... You know, dropping a manga isn't something one can announce. You just don't feel like reading an update of a story at that one time and then at another and you start to totally ignore its updates. Dropping just happens.
This story had a strong start and very good first chapters, but MC's apparent lack of intelligence becomes painfully obvious. And I'm not talking about genius levels of smartness. Just your everyday level. It's a bit hidden (kudos to the author to succeed in that), but if you thing about it, MC and the other characters dumbness in some important matters exists to create drama. Instead of hiding it, the author should just have found explanations for their behavior.
E.g. MC did only see one type of monster back then or maybe another type that lived near by and "might have joined the attack by chance". -> He wouldn't have known, that so many different types would come, which also explains, why he chose to go with a wall. He just didn't expect any flying types to show up. He'd also be right to believe the whole time a normal stampede would arrive. Alas, MC isn't stupid, but was set up by the circumstances. Much better. I should be editor at a manga label.