Understanding pass-or-correct responses is a fine advanced bidding concept that doesn't get the attention it deserves in North American bridge. (I have hopes that I will live long enough to play Wilkosz 2D in a live tournament. But so far I have only lived long enough to watch most of Poland quit playing it.)
If we're all going to share our favourite defences to 1NT, here is mine:
Double =
+
or
+
, at least 5-4
2
=
+
or
+
, at least 5-4
2
= both majors
2
, 2:spade = natural, 6+ suit (most often about the strength of a vulnerable weak two-bid)
2NT = both minors.
We call it "Chasm", for colour-shape-majors, since the 2D response in CRASH isn't GCC legal, and since it makes sense for higher bids to be more precise. The advances over double and 2C are pass or correct of course.
HELLO is indeed a strong contender in theory that doesn't get much use because of its complexity.
Oh, on the simple rusty 3C-Pass-5C hand, I make a nice quiet penalty double, unless my opponents are notorious underbidders.