stellar set is far more specific than just having a full set of 5* gear though. you must have a full (slots not needed and not counting sprite) 5* set, but with a gun and a bomb. most players do not seem to like bombs, maybe trying a few at 1*-3* and then writing them off as underpowered, because bombs scale badly to lower star lvls.
you also imply that anyone leaving or not at endgame burnt out. first, just because the game was out for a while doesnt mean that everyone started a while ago. secondly, getting T2 clearance is still accomplished very early in the game, just after you are given cobalt armor. you dont even have to beat snarby for it if I recall correctly, just get to 3-2 missions in general. a lot of people just try free games, then stop when they do not like them, trying to cater to these people can be useless if not downright detrimental.
also all of the numbers should be taken with a grain of salt, keep in mind that for example "conditioned response", arguably one of the easier achievements to do, is sitting at just over 8%