Not that the Packers would win a lot of games with Kizer playing, but I think putting an UDFA rookie QB in the #2 spot would be a disaster waiting to happen. No way should a team that is predicted to make a run at the Super Bowl have such a guy sitting as its backup QB. Might be fine for the Browns, but they have been playing for Draft position the last several years.
All of that leads me to the question of why even have Boyle as your #3? Only reason, is to protect him. Boyle should not be expected to take any meaningful snaps in 2018. If Rodgers or Kizer go down, time to sign a vet FA QB, if the Packers are still in the playoff conversation.