"Dramatically" isn't just a synonym for "rapidly" its proper synonyms are 'suddenly' or 'obviously' depending on context.
In this context it would mean that he gradually made improvements that resulted in an obvious (great) change.
I haven't read the novel but the Manga is fantastic so far...
@Alfredooo0s It's a really common trope in manhwa right now. I can think of a half-dozen manhwa and a lot of manga that have the formula.
"MC dies or fulfills a quest and is transported to the past" or "MC has future knowledge from playing a game or reading a book from his setting."