I've always assumed that the word
master is being used in the sense of "master craftsman" -- somebody who is supremely skilled in their craft -- as opposed to the word being used in the master/controller sense. Like how we call a great musician
maestro. So I'm going to go with either 장인
jang-in (craftsman) or 거장
geo-jang (virtuoso).
The old usage of maestro was, like sabum, to indicate a teacher. During the medieval and renaissance period, those who taught swordsmanship were referred to as maestro. The leader of a fencing salle is still referred to as maestro. I think the whole "master" thing is entirely a western thing, to be honest.
My guess that the English use of "master"/"grandmaster" to refer to high-ranking martial artists comes from chess, where those titles have been used for a long time to refer to highly skilled players. But that's just a guess.
The title "master" might also come from the more archaic use of the word to mean "teacher" (which I think is more common in the UK), though in that case, the top master is "headmaster", not "grandmaster".
The literal meaning of 'kwanjang' is ' head of the school' or 'head of the gym' so headmaster is probably the closest English translation. It can be (and is, by some) argued that if you own your own school, that makes you kwanjang regardless of rank. I know one local school that is run by a KKW 5th Dan who uses that term for himself. His instructor (a KKW 8th Dan about 2 hours from here) endorses the practice.
The 'nim' in sabumnim or kwanjangnim is, as I mentioned, an honorific, not part of the title.
Ultimately, I think that while it would be nice if the words were used correctly, it's not going to happen.
In our own school, the tradition is that the head of our branch of the Moo Duk Kwan is "Kwanjangnim" and the chief instructor in our program is "Sabumnim" while other instructors are referred to as "Sabum." This always makes me cringe just a little. Since adding the "nim" suffix implies respect, using the title for someone else without the "nim" implies a lack of respect. The "nim" is left off when referring to oneself, but never when referring to another (unless you're being intentionally rude).
I don't know that it's worth the struggle to correct the usage. Everybody knows how it's MEANT, even if it's incorrect.
If anybody cares, the correct term for assistant instructors would be either kyosa(nim) or busabum(nim).