This is trash! But a golden trash! Nobody mock this trash! This is gold, even if it's a trash!
Almost all isekai cliche development put together (reincarnation, OP MC, harem of beauty, secret organization, Demon, sword & magic, school life, etc.) You said it all, this manga has it. Such generic plot, nothing unique nor new. Search for another isekai with this unoriginal plot/arc, you can find a hundred. Just a mish-mash of every cliche act put together. This is why i said it a trash!
BUT!!!
I wont let anybody said anything bad to this trash! This is a golden trash. Mish-mash of every unoriginal plot, yet the author 'purposedly' make it to make a big joke for all of it!! And he/she dont forget to advance the 'serious' plot & act too! When its a joke, its a hilarious joke. When its a battle, its a serious and cool battle.
Almost like Gintama. But, in Gintama the MC and everyone there (except Shinpachi) know it is a joke and act foolish. In this one, nobody except MC think it is a joke/playing pretend. Everybody gamble their lives in shadow garden, ready to die to bring down Order of Diabolos. Just MC and only MC is thinking that this a bunch of semi-adult people playing pretend & play along with him in exchange for his isekaied knowledge to gain money & train them.
The one big joke in this manga is MC & his underling lifelong misunderstanding. Its so unscientific, almost impossible to happen. And for so long, many many years... without neither of them found it out. Yet we can just laugh at the result of it. In another isekai manga, i will criticize this as illogical, impossible, and lazy script writing. But in this one, it is the joke. I think i will lose and branded as someone who can't take a joke and 'must be a fun guy at a party' if i point it out, tho.