I'm gonna ask you this simple question. Have you even played a Halo game for a substantial amount of time?
Chief fits more so than Snake. Human in power armour. Sounds familiar...
Halo 5 was rated T. Even then Halo is an extremely light M rating. The thing that pushes it up more often than not is the Flood.
No comedy or light heartedness? Because Grunts flying through the air when their methane packs are broken is so dark and edgy. Because Chief saying "Boo!" to a Grunt is really dark. DJ Brutes. Random dialogue. There are loads of examples of light heartedness and comedy in the Halo games.
We've discussed how guns are not a problem for the Chief. Power weapons (Rocket Launcher) + AR with Kinetic Bolts add-on.