what is the temp of a dry ice ethanol slurry? see Ice bath tech - #16 by cyclopath
50:50 glycol will become slushy and hard to pump somewhere between -30 and -45C especially if the bath it’s in is at -70C or better.
methanol, ethanol, isopropanol and acetone are all usable heat transfer fluids at -50C. HC-50 (potassium formate) is the only non-flammable one I’m aware of. At -80C I believe your choices start to get pretty limited.