So, there is this mod I've been using a lot lately, called the New TARDIS mod. One thing it adds is a "Space" Dimension, which has no oxygen. So, my suggestion is this; if you have the system here that allows you to breath underwater, it let's you breath in space, for no cost! (Since the space suit magicly provides infinite air).

A second part exists for the future. When Modular Power Suits, New TARDIS and Advanced Rocketry are all on the same version (Meaning when Advanced Rocketry updates to 1.16.5), then this is compatible with Advanced Rocketry's oxygen, and then it uses that oxygen in New TARDIS space!

Just a suggestion though!