>Seems like ice is a better choice? The phase transition requires a huge amount of energy.

Why are you excluding the phase transition energy of the brine? Presumably you can calibrate the salinity so it freezes at around say... -15C, and set your freezer to -18C. That way it's absorbing heat more heat when it's colder, which is helpful if you want to keep your food frozen. The latent heat of water isn't of use to you if all your meat is already thawed.

Because OP mentioned saturated brine and a freezing point of -18C.

That's the point! The brine is frozen at -19c and liquid at - 17c which is well below the freezing point of everything else in the freezer.

Or you can have normal water which is frozen at -1c and liquid at 1c. Your pick.

The handy thing about the brine is that its absorbs heat at a temperature below that which everything else thaws. So as the brine thaws it keeps your freezer at -18c which means everything in it is still frozen.

But if the brine is not frozen at the working temperature of your freezer, it's completely ineffective compared to water.

Of course, you need to choose a salt % that gives a melting point just above the temperature you want to keep everything else, so that the salt water is normally frozen.

I mentioned -18C because that is the recommended minimum temperature for freezers such that some foods can last almost indefinitely; certain chemical reactions are completely shut down.