Sorry I don't have anything off the top of my head, but I would recommend looking for a novel with these three tags. Overpowered Protagonist Hiding True Abilities Underestimated Protagonist Hiding true abilities is optional, but if you don't have it you'll probably get a lot of novels where the underestimated part comes from a sudden strength gain. Like those cultivation novels where the protagonist starts out as trash, then suddenly gets a divine technique and becomes overpowered.
A dragon in slimes clothing I said make my abilities average martial kings retired life The New Gate the other side of the last boss will the re-summoned hero live as an ordinary person (yes he will )
World record (closest novel to opm for now) Mahouka (latest volume his power started being revealed) Evil god/The other side of the last boss Instant Death
I know you said novel, but I'll put this here just in case... Mob Psycho 100 -Another manga by ONE, author of One Punch Man. It has an anime, too.