The last two Kumodesuga books were originally released a mere month apart, a release schedule the English translations unfortunately did not share.
The fifteenth and penultimate novel is mostly exposition to set the stage and get the reincarnations up to speed… so I can certainly see why the author didn’t want to have a months-long wait between it and the series’ conclusion. Taken together both novels wrap everything up well enough without much in the way to complain about besides a rather arbitrary ending to the climatic battle and some fairly half-hearted character epilogues.
When all is said and done I think the best part of this series ends up being the middle segment, after the Spider and Demon Lord start traveling together and before they launch the war against humanity. Which is to say, roughly, volumes 6 through 10.