Before putting an iPhone5 into very long-term storage should I fully drain battery? I just migrated an iPhone5 to an X. I'd like to put the iPhone5 into very long-term (probably 1+ year) storage. While I'll hopefully never need it again, I'd like to keep a back-up iPhone at the ready.

Given the chemistry of iPhone batteries, before putting it in a safe place for a very long time, should I drain the battery to 0%? or 100% charge, and power-off?

It is fresh off of a factory reset, so there is no data to be lost on the phone.
When it comes to Apple products and batteries I recommend users refer to what Apple has to say about its batteries.
However, to answer your question, I quote directly from Apple:

Store it half-charged when you store it long term.
If you want to store your device long term, two key factors will
affect the overall health of your battery: the environmental
temperature and the percentage of charge on the battery when it’s
powered down for storage. Therefore, we recommend the following:

Do not fully charge or fully discharge your device’s battery — charge it to around 50%. If you store a device when its battery is
fully discharged, the battery could fall into a deep discharge state,
which renders it incapable of holding a charge. Conversely, if you
store it fully charged for an extended period of time, the battery may
lose some capacity, leading to shorter battery life.
Power down the device to avoid additional battery use.
Place your device in a cool, moisture-free environment that’s less than 90° F (32° C).
If you plan to store your device for longer than six months, charge it to 50% every six months.

Depending on how long you store your device, it may be in a
low-battery state when you remove it from long-term storage. After
it’s removed from storage, it may require 20 minutes of charging with
the original adapter before you can use it.