So the pacing is off, the premise make no sense (I don't get why isekai put so much emphasis on guns they aren't better than calling down lightning like you're freaking Zeus), and the Einstein reveal is just nonsense. Not because of how weirdly it was framed but the type of scientist Einstein was. He was significantly more focused on theoretical physics, quantum theory, etc. That's not to say he didn't know practical science, but if anything he's the type of person to get giddy over the thought of divine powers and reincarnation and try to study existence as a whole. Furthering a war and driving those people out is the last thing he would do. A simple fix would be to replace Einstein with Thomas Edison. It'd still be trash, but more believable trash. Edison was more of a cutthroat businessman and didn't hesitate to borrow ideas or turn the public against others (cries in Tesla) to further his success.
This really just reads like some random fever dream fan fiction from a Japanese person whose vaguely aware that Albert Einstein was smart and that's it.