I was kinda more touching on the fact that the story shifts so much as the series progresses. Without spoiling, book 1 (Reaper) serves as a "prequel" story of sorts, establishing some of the main characters and the world they live in, the main conflict, etc. It also covers a lot of Uzuki and Mitsuhiro's relationship while they're trying to find out how to stop themselves from dying (as their lifespans are marked on Mitsuhiro's body). The big conflict that encompasses a large part of the story isn't even resolved until Book 3. But by book 2, we introduce time travel, space and matter manipulation, demon possession, government corruption, dark demon overlords, etc. and in comparison to the first book, I feel like I'll be whiplashing my readers a lot xD"""
The main conflict changes again after book 3 as well, with the antagonist changing and the world being at the hands of an even worse baddie than in books 1-3. It's all still relevant though, I'm not just throwing enemies in to keep the plot going lol It's trying to keep my readers actually reading through it without going "welp this story has done a 180" and it not being what they originally loved about the first book to begin with - that's the challenge.
I get where you're coming from too though. I've thankfully been able to do almost 1 chapter a week, now that I'm done of school, but back when I was in school and working at Starbucks at the same time, I was only able to do one page update a day, save for weekends, and it was just so aggravating that it took us 4+ months to get to chapter 4 >:I whereas now I did 4 chapters in like, the past month and a half lol But baby steps I guess :U