

I’ve been using Google Fi for the last few Pixel phones I’ve been on. Each time I’ve switched, during setup I just get asked if I’d like to activate the phone (with a warning that my old one will be deactivated) and I click yes. It’s then active before I can even complete the phone’s OOBE setup.
Android also does apparently have a “copy to another device” function mentioned here, but they hint to what you said regarding carrier limitations applying. Though Apple’s quick eSIM transfer has a similar note as well.
Definitely seems like a “When it all lines up, it can be convenient, but when it doesn’t you’re fucked” situation.

I learned a bit ago that if you hit Ctrl+Alt+Delete seven times very quickly, it’ll bypass a stop task that is being pesky like this!