Re: Modern army meets fantasy

This one is exactly like what you are looking for. Less of a focus on a single character, the main character in the story could be considered to be America instead of individual person.

Translated stories, which I know exist and have read some, but eventually dropped:

A dungeon core type story where dungeon makes technology like guns in a fantasy theme world

You may or may not like this, it has the whole developed tech goes to undeveloped tech world theme, but for cultivation world type technology

I read a few chapters of this and didn't like it, but it fits your criteria, so maybe give it a try