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Fun, enjoyable, and exactly what I wanted
Note: I've already watched the anime adaptation of Tensura Nikki and have also read the entire main series manga (as far as English releases are concerned) + watched the first two seasons of the anime
Tensura Nikki is a fun 4-koma spin-off of the main Tensura series, with each set of chapters in this volume roughly corresponding to a specific time of year. It focuses on daily life, the way Rimuru imports festivals into the new world he's found himself in, and other more mundane aspects of the world.
Naturally, most of the jokes are in-jokes that require understanding each of the characters involves, their traits, and their relationships.
From a language perspective, I found this to be surprisingly difficult (note, coming from the perspective who read the main series in English rather than Japanese). Several characters use language styles that are completely irrelevant to modern Japanese (notably Gabiru, though his language is so narcissistically arrogant and self-promoting that it's endearing, and Ranga, who is incredibly deferential to Rimuru), including dialectal forms (Kurobe probably gets more screen time here than in the main series) and elderly forms (Hakuro). This all results in a wide variety of language, much of which is either archaic or poetic, but which is also necessary to understand and enjoy the work.
It's also important to note that due to this volume focusing heavily on Japanese festivals, tons of festival-specific vocabulary appears, and the writer does not hesitate to be poetic or flowery at times, especially in panel titles.
Overall, due to a lot of the language being fairly unlikely to be of any use outside of similar types of literature, from a language study perspective this should probably not be your first choice. However, if you're already at a JLPT N2/N1 level, you should be able to enjoy this and only need a dictionary for every few pages when the random kanji variant, proverb, or kanji so small that you need to squint to try and make it out appears.