If the 5 seconds are a little too much on some maps where spawns could be next to where survivors may camp, this is something worth tweaking. Another option is to change the god mode to be a form where you cannot take damage but are still affected by physics force (explosions, etc).
Is there a way to make it so that zombies are invulnerable while within their spawn zone (only) for 5 seconds? So if they step out of it they immediately lose it? Some maps have very close zombie spawns so they can start attacking while still invulnerable. This is especially true when using scouts or speed-boosting perks like Leap, Swarming, and Rage.