Looks like Renekton is still a trash tier champion
Renekton's issue late game are not his numbers but rather his kit. So instead of artificially trying to buff his late game (which will do nothing for him) why not try and buff his early game a little so he can perform better against certain champions that are supposed to be "late game" ?
Just give his Q +5 damage on level 1 and remove 1 second from his 18 second cd gap closer and improve him from there. Giving him +30 damage on emp Q at 20th minute does nothing, he either lost or won the game by that point.