Oh, I thought it is from 21% to 40% because I suggested before that +1 attack stage gets added every 20% difficulty but you said it already does that:
If you accept it, that way stages ordering will have more distance of appearing instead of only 10% as you mentioned above:
Because one stage of attacks until 50% is just too easy.
Oops! I just realised that Egg Cannon doesn’t appear from 0% difficulty. What difficulty doesn’t it start appearing from then?
Could you add messages saying “Please wait.” at
? And add a message saying that you reached the end of the list when clicking
or 
At Heavy Metal wave when trying to get its pecking order you have to wait for the barriers to spin a while spin slowly again, how about you reverse the pecking order? So it be less boring to get.