A hacker doesn't need to clone your SIM, all they need is access to an SS7 line almost anywhere in the world and they can see your messages, regardless of carrier or phone. I suppose North Korea probably doesn't have access to SS7 servers, but that might just be the only one. Granted, SS7 isn't cheap or easy to get access to, but when it comes to banking fraud, the economics change.

The victim will be disconnected from the network, but there's no way in hell the first line of carrier support will detect any of this. You'll have to put your faith in the security monitoring of your carrier (the ones letting spoofed numbers in and out of the network, so good luck I guess). There's absolutely nothing you can do about this thread other than hope that your carrier is smart enough and that you're not important enough for a sophisticated fraudster to target.

As for cheaper threads, everyone who tweeted about owning a crypto exchange account with their phone number on display will probably lose their SIM at some point. SIM swapping is easy with a fake ID, and people within phone stores have been caught doing it from the inside.

SMS is insecure and often abused. Don't use it. Maybe also disable 2G on your phone while you're at it.