^^^ Right on.
Is this a matter of Chekov making Captain (by rank) or Chekov making Captain (as in, commanding a ship)?
They're not necessarily the same thing.
As mentioned, Scotty became a Captain by rank but didn't command a ship of his own. Perhaps Chekov also should have had Captain's rank by his last appearance.
As far as commanding his own starship, yeah, maybe he was content to stay at the helm or weapons station or security (he wore a lot of hats).
But then again, he was XO and seemingly on the command track. But never got his own command. He changed his mind?