...Okay, it might be silly to complain about biology mistakes in a manga about giant invertebrates, but: stink bugs (family Pentatomidae) and assassin bugs (family Reduviidae) are completely different. Not only that, but most stink bugs are herbivores.
This really is an odd isekai story. It has many of the usual cliches like OP protagonist, slave harem and royalty with an implied dark secret, but also a protagonist with genuine friends.
"there could be larvae burrowed inside."
Nymphs. Cicadas undergo incomplete metamorphosis (they don't have a cocoon, basically), so their juvenile stage is a nymph rather than a larva.
Rereading early chapters after the later ones is extremely rewarding... you get to see all sorts of foreshadowing, like a certain character not telling the whole truth.
"I might be a lolicon, but I think things might work out with you."
As far as lines I never expected to hear in a manga (or anywhere) go... it's not actually the weirdest.
The part of this subplot that really annoys me is the failed attempt at a "porn stars are people too" message. Sure, this was shown at first with Airi wearing normal clothing and suffering from jet-lag... then she started molesting her employer.
Go-toubun no Hanayome, double the size, 18+ edition.
While there are undoubtedly series that have a more similar premise, I picked Go-toubun for this comparison because of the similarity in names and the harem also all being big-breasted (with no obligatory loli).
Why don't the people of this world know at least basic arithmetic? It's not that hard to teach, and it's an essential skill for pretty much everything.
The thing about Youta's logic in this chapter is that it's actually not pragmatic at all. He claims that, in a world where he's the only man, he should keep the girls safe, when it's actually the other way around. As the only man in the world, he is (as far as he knows) literally irreplaceable...