It may or may not be balanced currently, but I wouldn't balance it based on what feels right when cheating with the creative menu, either. And I'm not sure why you bring up glitches in A16 - obviously glitches shouldn't factor into the balance, either. If you want to min-max your gamestage, I would suggest getting your heat/activity map piping hot so that screamers are coming all the time. Or much easier, turn up your XP gain in the settings.
It's admittedly weird to have achievements based on gamestage, because gamestage is a non-obvious stat that would typically be calculated behind the scenes. As I recall, there's no UI telling you what your gamestage is outside of the console. So from that perspective, tying it instead to zombie kills makes sense. On the other hand, there are already achievements for killing zombies. We've also definitely had Alphas where players complained that killing zombies was the "only" (read: optimal) way to get XP and level up. So that faction that likes to focus on the non-combat parts of the game may not appreciate tying more achievements to kills.
I guess gamestage may be the best thing that can be quantified for a 'reached the endgame' type of achievement, until the game has a real story and a real endgame. And I'm okay with achievements taking a long grind to achieve, and/or achievements that can't be earned until the game goes gold. After all, you can only earn them once.