You said it yourself, TWOK is about kirk's midlife crisis, with the narrative coming from khan's desire for revenge. Genesis is a smaller part of the plot, is is mainly there for kirk's son to argue with him, at the end of the film the genisis subplot isn't really resolved because that's not what the movie is about. I don't see how Kirk finding out he has a son while battling an enemy that he has fought before, is going where no one has gone before.Star Trek II: The Wrath of Khan.
It was a vast improvement from TMP which was something other than Star Trek IMO. The movie had more going for it with Kirk steering the narrative; his midlife crisis, an adversary who won't rest until he gets vengeance, a project which can save lives but in the wrong hands can be used as a weapon of mass destruction. Spock has an eager student who wants to be a great officer, is now under Kirk's wing--she shared his failure, his triumph, and his tragedy in one great space adventure.
What more of an example of a film which represented what TOS was at it's core? Action, some drama, laughter, and suspense. I didn't say II was the best Trek film, that falls under The Voyage Home; but this thread is not posted as a countdown list which some has done. ST II: The Wrath of Khan is my favorite because it's a base where a good writer and producer could go anywhere with it.
There's always possibilities and that didn't mean The Search for Spock and another crap hazard called The Final Frontier, but the possibilities of seeing where those characters engage to boldly go where no man (This also meant women for those who don't know) has gone before.
TOS had its fare share of action stories. But it also had plenty of episodes with a slow moving plot, that had the drama come from the unknown (aliens, or being that the crew couldn't explain that they had to understand in order to overcome).