If instead of merging and compressing worlds into one planet, he made multiple planets in a singe or more solar systems (since there arent any aliens in that universe) it could have been more logical.

Its hard to imagine thousands upon thousands of planets merging into one. I mean where do you put all that land and sea, not to mention population. If one fused that many worlds, its impossible to compress and will result in a planet billions of times larger than a super nova. Its more logical to just destroy the dimensional barriers separating them and join in one universe butdifferent planets instead.

Do you really think he became a dropout due to that? REVEALED PLOT ISN'T ALWAYS THE TRUTH.

Supposedly the 'monsters' are immune to the original rules of the world (physics) in that they are supposed to be immune to human innovation and science, being the fantasy creatures that they are. They even explicitly state that not even nuclear weapons would be good enough to stop them. However, the MC builds a pile-bunker (well within the reaches of human understanding/knowledge/physics might I add, nothing magical about a damn pile bunker) and uses it to easily squash and one-shot high-leveled mobs

MC spends only 250 years to read every single book in the world. On top of this he traveled to every country in the world and just about learned every single language. You have to consider that, even if this was possible, you'd have to be some kind of freakishly fast speed runner with a perfect memory and a incredibly powerful comprehension (i.e. You basically have to be a genius). Despite this though, the MC still acts somewhat braindead at times and you often question how smart he really is.

What's also hilarious is that he spends an additional 50-100 years learning how to be a blacksmith, because apparently forging swords trumps nukes. And apparently, learning how to become a master blacksmith takes a quarter of the time it would take to learn all the languages, read the books of, and visit the countries of the world.

There is no explanation, or hints about why the MC was left behind and thus unable to learn how to control mana. There is no explanation why the angel that visited him could not teach him mana control in his own world, or why building up his body when even nuclear weapons supposedly won't work against these mobs, what is a karate chop or a round house going to help? Basically the MC memed his way into becoming a proverbial Chuck Norris and now somehow he can compete with people that have powers that allow them to surpass nuclear weapons (checkmate, Gandhi).