> The reverse might be theoretically possible during activation (if you feed the QR code to your own custom device and your software implementation doesn't guard the keys, letting you copy those freely), but I suspect there is some form of attestation so it won't let you activate with anything but "trusted" devices (by network, not yourself).

There are programmable SIM cards available that allow you to install an eSIM from a QR code. Those SIM cards can then be used in any phone that doesn't officially support eSIMs: https://esim.me/