having a traditional FPS camera on top as a toggle would void one of the main advantages of this system: the fact that you don’t have to build 2 different animation sets+systems.
for a true first person to work (specially if guns are involved) you’re going to want the animations to be stabilized to a certain degree as well. if you rely only in a code-based solution you can easily end up with a stable camera that sits on top of an unstable character which would lead to viewing “wrong” parts of the character very frequently (inside of the neck or arms, etc), and having the arms not properly positioned on-screen and jerking all around.
for my last game (UE3-based) I implemented my own code-based stabilization for the true first person camera, as well as fine-tuning all third-person animations to work in true first person. it worked well enough for an indie game just as it would work for an adventure or parkour game, but it’s not quite there from what you’d expect from a AAA shooter