Regarding the oxygen supply: I usually tunnel towards a colder area and build a self-powering oxygen machine there. It’s basically a room with electroylzers and gas pumps at the top and bottom (or a gas filter) to separate the oxygen and hydrogen. The hydrogen then goes to hydrogen generators, the oxygen is pumped into the base. Another important bit is to make sure the CO2 has a place to go (downward) before you can deal with it.