If we had the tech in theory could we not say bypass breathing with a device that oxygenates blood then fill all air gaps with a liquid/gel that's made for it's density and to be non harmful instead of focusing on breathability.

After that maybe a flight suit to contain/ keep separate the different liquids/gels. As well as maybe a liquid only feeding tube as well as what ever you will need to capture the "waste" after lol. Things needed for extended use.

I like to invent hypothetical things and try to find ways to make them real and this is a fun one, out of my wheel house to attempt though even if I still had my workshop sadly. Oh how I miss trying to build a food generator for real.