Instead of finding caves, a good bet would be to send nuclear powered robot tunneling machines on a one-way trip. They wouldn’t have to be fast, only methodical.
Every few feet into the hard rock, they would insert lightweight advanced ceramic reinforcing rod into the ceiling. When the tunnel was mined, then spray the inside with sealant against micro fissures.
Then their landing craft could be cannibalized to make ceiling, wall and flooring, as well as pressure doors. Once done, it could even be pressure tested before the humans arrive.
This would give lots of space, avoid the temperature extremes and lack of atmosphere, and any cosmic or induced radiation. The nuclear reactor powering the robots would remain for the energy and communications needs of the Mars base.
The technique could be perfected on the Moon, first, again, without people.
I often wonder if Mars can ever be terraformed. The main problem is that the planet’s core seems to be “dead” and can’t generate an adequate manetic field.
So I guess the next step would be to “geohex” the planet. With the help of an army or robots, simply build a small dome structure, terraform the dome, then build a connecting dome and redo the process. Rinse and repeat until you get the desired size.