The logic behind strengthening perception is because MC is primarily a firearms user, so perception greatly helps with his accuracy and dodging. He supplements his lack of strength with advanced weaponry, and in most parts of the story, even if he increased his defense, most of his opponents would still tear through him like wet paper.
Smarter doesn't mean wiser. It just means he can think faster, but he can definitely still make bad decisions. In fact he is very unlearned in quite a few areas, which he leaves his subordinates to cover.
He doesn't collect battle spoils because he has no way of using them. Most of the tanks are damaged and he can't repair them, he has no way of producing ammo for the guns of his enemies and he needs money way more than weapons because of his serious debt, and he literally has no opportunity to collect slaves most of the time. However he actually does use some later on in Volume 4. The whole reason he can even sell them is because his organization is researching the tech, not actually using the gear.