Get a freezer that works down to a realistic low temperature, not just a measly -18C, package some unflavored high-strength PG and VG well, and crank that freezer down low. The liquid is probably good for at least 10 years if kept very cold, and maybe longer. The PG won't freeze but should keep well. You'll also need a generator and diesel to get past power outages. Don't use a gasoline (petrol) generator as the fuel doesn't store well, so diesel or LPG (propane) is best. You can use a gas (LPG) powered genny, which can be adapted from a petrol one such as a Honda suitcase genny without too much bother, and the propane gas tanks are easily stored. Or green power - with solar panels plus a wind generator, a decent battery bank and an inverter you can create your own power much of the time. If you have 3 power sources of this type you can be independent if your needs are not too great.
Well, I guess that is all kind of weird for the average city dweller, but many of us don't live in cities.
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Also note that if you really want to a freezer to work properly then don't use a regular store-bought one, use a marine system self-build. Store bought freezers are made to a price, they are made to look good, and above all they are made cheap so that a constant large amount of power is needed, as they only have about 2" of insulation. This is not the way to make an efficient freezer.
Instead go the marine route, where a low temperature without huge amounts of constant power is the defining function: get a good Danfoss compressor kit that can go down to way lower than -18C, then build a box much bigger than normal so that there is space for 8" to 12" of insulation, create a fibreglass internal liner, then foam out the gap between the liner and the outer box with insulation. You can use foam board or spray for this. There are some details you need to get from a proper design, such as where to put anti-damp foil sheet, how to create a drain pipe that doesn't leak heat, and how to make the lid correctly. This type of freezer stays colder for longer with far less power demand. It is generally built into an odd space in a boat, using the boat hull as the outer skin, and it is more efficient than a home freezer. Using cold air leakover you can also create a fridge next to it, with no additional components, by leaking some cold air across into another well-insulated compartment.
