One of the only reasons why the story of The Eminence In Shadow works with an "overpowered" protagonist, is Cid's in universe willingness to go along with the flow of the story.
As he has demonstrated time and time again, he has enough raw power, technique, skills, to just completely obliterate any challenges that come across his path. but, when he doesn't really have to do anything, he keeps playing along with it, playing out his childish fantasies for the sake of his own internal "plot" .
as we see in the bomb collar arc (in the manga and light novels, anime onlys be warned), he has enough power to cause an integer overflow on the collar, but he pretends to be a detective conan-like character to help out who he thinks are the "main characters" because he felt like it was a fun role to play.
this is consistent with his earlier behavior, such as when he was playing the role of a disgraceful, unorthodox swordsman, to fulfill his fantasy of fulfilling the classic "crouching moron, hidden badass" trope in a tournament. he spends all this time acting as an eventual antagonist for other characters to study and train for, showing hints of his "true" power (lmao), going along with the classic tropes, flowing with the "story" he thinks he is in, instead of the one that happens with him as the focal point.
these are my reasons why i think Cid is a great example of the overpowered main character trope done right, and i hope you have a good day.
