What an obnoxiously contrived setup. I hate characters who have no idea how strong they are when they very clearly should. This guy was a clerk two years ago and met adventurers constantly, but has no scale of how strong an E-rank is. He also has no idea he can crush slime bags and explode the arena with his bare hands, but surely that strength would've been made apparent in his training so long as he wasn't just sitting in a magical metal box that he couldn't break. Heck, he even knows in the flashback a slimebag can apparently take a dragon's tooth and survive. I guess he just forgot. He keeps comparing the world and people around him to his party, which you could maybe use as an excuse if there was even a hint that his party was lying to him about their strength or keeping him in the dark but it is public knowledge the party he trained with is the strongest on the continent.
it's such a stupid fucking setup. It makes the mc look like he's an absolute idiot and the only reason I can think of for writing it this way is so you have an excuse to explain the world to the audience. If you have to make your MC have as much knowledge of his own world as someone who just got isekai'd into it yesterday, you've fucked up. That's not a character, it's a wall of meat.