Safest way to activate a "backup" iPhone, to be kept on a lower version of iOS I have a Jailbroken iPhone 6S on iOS 12.1.1, and I am really, really happy with my current setup. I want to stay on this version of iOS for as long as I possibly can—several years at minimum.

I have taken the somewhat drastic step of purchasing a "backup" iPhone SE from Apple, which I will keep on (the lowest possible version of) iOS 12, in case anything happens to my primary iPhone.

I've heard stories of Apple not allowing activation on iPhones with too old a firmware, so I assume I should activate my new SE as soon as it arrives. If I use my primary phone's SIM card to activate the SE, is there any chance of my primary phone becoming deactivated? Are there any other risks I should be aware of?
Don't worry , you can use the SIM card to activate the iPhone SE.

You can always make backup copies, regardless of the iOS version you have. An important fact is that in order to restore an old version, the version you are going to restore must still be signed by Apple. Normally they leave the 2 most recent versions available, in these moments iOS 12.2 and iOS 12.1.5.

You do not have to worry, they will not remove the Jailbreak or anything like that by activating another iPhone.