The Ultimate Way Proximity Chat Is Bugged (on Console, atleast) :

Proximity Chat is an amazing feature that has been added into Marathon, however there is one serious issue. On PS5 (the platform i've been playing), you *CANNOT* hear the players in Game Chat (whether through proximity chat, or in your-ingame party) whilst you are in a voice chat such as a PlayStation Party or Discord.
This really affects the experience, as literally many players like to stay in Party/Discord Voice Chat when playing with their friends, but switch to Game Chat when they see other players have their mics on and are nearby. By not being able to hear them while in Party Voice Chat, it really removes a big part of the way people interact with Proximity Chat.

Essentially, Hearing players voices in game even while in a Party Chat/Discord chat on Console NEEDS to be fixed/added. It will really improve the gameplay and proximity chat experience.