My single brain cell is killing itself.

Why are you here?

Why are you pretending to be an undertale person?

Seamoth would be your best bet. The seiling of the cave should be just above 200m mark in most places. Building something is just not worth it. Though, maybe explore Aurora first and come back there later.

Sparre o2 tanks

I advise going to the aurora to take care of the radiation problem and the get the prawn. Then go to the underwater islands to get the grappler, and the drill for magnetite

My tips:

A- use a floating air pump and pipes

B- raw bladderfish

C- vehicles/seabases

That sounds like an interesting challenge -- tackling the Jellyshroom Degasi base in the way you decribed.

First thing that comes to mind are power transmitters, which would allow for successive exploration, connecting Oxygen supply bases to each other, energized by a single source. A pipe system might be viable, alas, since they only emit Oygen at their very end, and I do not know, if it's possible to reconnect opened pipe segments, I'd first go for some experiments. No use in a 250 metre pipe, if I need a breath of fresh air at 200 metres :)

At any rate, I'd try and go for the entrance directly above the Degasi base.

My approach to unknown terrain always was, to make perfectly sure I knew my way back. If in doubt, turn around. The usual labyrinth tactics apply: Take only lefts, take only rights, only up, only down, etc. Next time I'd reach that point in about two thirds of the time or even less, and ideally with a comfortable Oxygen reserve.

But as others (and you yourself) have noted, there's nothing better than having a source of Oxygen to fallback to, if things go tits up. No use in having more inventory space for loot to pick up, if you can't bring it back.

Ultra High Capacity tank *2 and seamoth that way you can easily explore the area