Zenos was fine as a one-expac villain. Having no particular motive beyond "find a strong enemy and get a good fight" was a fine contrast to previous "well-intentioned extremist" antagonists like Gaius or Regula. The problem is that he never actually does anything to justify coming back. He stalks the WoL for a while, then when he finds out we're out of his reach now after going to the First, he suddenly decides that he wants his body back. Then he becomes the writers' catspaw to start writing Garlemald out of the story. His entire arc in Endwalker is literally throwing in his face that he's irrelevant.

As far as a rival? I never felt it. He was a roadblock, then he was an annoyance, then he was Fandaniel's pawn, then he was a loose end to tie up. I never felt like he had the narrative justification to call himself a real rival.