You could also use clonezilla. The arch install media comes with it. It is handy, because you should not boot in the system you want to clone anyway. So you can boot into your arch install media and then do all the disk cloning.
Funnily I also have to do the same thing (from 1TB M.2 to 2TB M.2) on my desktop. I might do this tomorrow or on Sunday. I plan to do this as I described. Well I don't need to save a disk image on an external drive like you because I have two M.2 slots on my mother board.
Maybe write a follow up here how you did it and if everything went well?
Good luck!
Edit: Hey my first Lemmy comment! :)