If using Galden or any other perflourocarbon you cannot use electrolytic capacitors with a rubber bung, as this will swell with time. Same for wire, the only wire stable long term is PTFE insulated, or some Kynar wires, and you will have to avoid any ABS or other susceptible plastics, as they will eventually degrade with time or leach out into the fluid.
What is wrong with mineral oils, you can get them with wax temperatures below -55C, though in that case they tend to be more like JP1 rocket fuel with a similar viscosity to water. Same caveats as to compatibility as well, but at least the JP1 will be cheaper as well, though will wreak havoc if you have any controlled impedances on the board or fast signals ( will increase loss and capacitance), or just use regular Jet A1, which will at least not be solid at -55C, though it will probably be a softish wax till it melts, which can be an advantage as it will sink heat nicely in the phase change. You will need to have a dewatering agent in there though at the bottom, as it will have some water in it, though boiling it and getting the water out will help, but you will want a nice big pack of water absorbing molecular sieve in there as well at the low point of the tank.