With the scale of the story widening, I think that a better story arc would have been the reveal that the Kingdom was preparing for war with a neighbour. There's a lot more potential for different plot lines there than everything being the fault of some greedy jagoff. Once the story expands to that international level reveal that the reason the other nation was threatening war was out of desperation due to a famine, or summat. Throw in a few intractable jerks in power along the way, of course, and have tension between different outcomes as the MC tries to save everybody. We can then have the big forest run out of monsters to hunt, or something, to bring the story back into a more local focus. The mangaka can milk these sweeping arcs for several years this way.
despite Japan having some leftover feudal relationships remaining
The trope in manga that I've always found weird is having the last name of the ruling family be synonymous with the territory, whether it's the entire country or something sub-national. Historically, around the world, most royalty or even nobility just used first names, since it's rather assumed that there's only one King Charles III that you're talking about. This is true in Japan as well. I'm pretty sure that never once in his life has The Reigning Emperor Naruhito ever introduced himself as Naruhito Japan. I highly doubt that his birth certificate has anything other than his given name. His younger sister didn't acquire a last name until her marriage in 2005, and that meant leaving the Imperial Family, so there was never a chance that her full name would ever be Sayako Japan.