Huh bit unfortunate I didn’t see this sooner. But I think your initial suggestion is a pretty solid one!
However I would suggest some revisions to both simplify the explanation and cover some potential problems.
1) Patient zero maintains the enraged buff until 25% of the lobby is infected.
2) Enraged will be/continue to be granted to the lone infected until there is more than one infected in play.
The second clause should provide a failsafe against people leaving and causing the percentage to drop below the 25% threshold again, triggering enraged. This is because it would be possible for somebody to leave the lobby, thus letting patient zero get enraged again, then rejoin the lobby with a buffed patient zero. Basically it’s a bit of protection against people trying to metagame.
However, it should also provide a failsafe from patient zero infecting one person, one of them leaving, and now the other is stuck trying to take on the entire server alone without enraged.
As another tack and slight revision to your ideas for patient zero enraged buffs. Perhaps it could be something like:
1) Upon being enraged, every death grants an increase of 20 HP.
At the present moment it takes only 10 Tommy Gun bullets to kill enraged, put that up against a lobby of 11 people spewing out >100 bullets per second and that really isn’t enough. But adding 20 HP every time will make it so that enraged becomes progressively harder to stop and rapidly killing them repeatedly will quickly make them resistant the volume of fire thrown at them. For example if patient zero was killed 11 times (initial spawn, then 10 deaths after enraged) they will have 400 HP, double that of the base enraged’s health. Even if they get hit with two plasma rifle alt-fires they’ll still be standing.