On the one hand, yes, you're right, it would probably be too much too soon. On the other hand, the more "obvious" it is that Anna's mysterious secret is that she's actually a robot-girl, the more amazing the reveal will be.
The fanbase will be all, oh man Anna's so obviously a robot you guys, she's barely hiding it, the near monotone and terse answers, the inhuman reflexes and strength, the simul-texting, the effortless hacking, the crazy sensor-ing and multi-tasking, the oh-so-mysterious-and-classified meetings with the instructors, the complete naivete and ignorance towards almost everything not directly related to combat, the way she takes things way too literally, the "I knew nothing good would come of fighting humans", the alphabetical order...!
And the fandom's sense of smug totally-called-it-ness only increases when the studio leaks that a movie is in production which will center around and explain "the history of a certain popular character" and, wouldn't you look at it, a trademark was just filed for "Project Valkyrie Core: The Movie: Anna Sanchez: The Ginnungagap Project"
And then the trailer for the movie is released and it's mostly scenes of a certain frame that is very clearly Durga except with a fully opaque helm and, actually, no sign of exposed human flesh at all, really, going full Dynasty-Warriors-Lu-Bu through literal armies of Antagonists, but in between a snippet of Anna's duel with Sekhmet over the mountains of Canada and a visibly battered Durga flying solo into a giant glowing green jagged rip in the sky, is a scene of two U.N. generals (one of whom might possibly be totally-not-General-Revil) in some sort of command room, staring at something past the camera, and one of them says "The Ginnungagap Project. With this... humanity might just stand a chance of winning this war." Possibly with photos of Anna in the background next to incomprehensibly dense physics equations.
And the fanbase feels completely vindicated because this "Ginnungagap Project" is obviously the secret robot super weapon project that created Anna Sanchez and holy shit the hype is real-
And the movie opens to a tiny, cheerful red-headed girl playing in the Canadian wilderness who stumbles over an odd metal sphere, and when she places her hand on it it glows (with Tron-lines, probably) and phases into her...
And the audience's confusion - because this is not what they expected at all - at the adorable slice-of-life scenes (Cute li'l Anna playing with her family and friends! Cute li'l Anna hovering unsteadily on old-timey propeller wings! Cute li'l Anna with a massively oversized backpack and an adorably determined look on her face fixing telephone poles 'n stuff!) gives way to slowly dawning horror as they realize what has to happen for Anna to turn out the way she did - and the sharper viewers start to remember times when Yukari-nee mentioned "The Alaskan Offensive, that took back a foothold in North America, just last year"...
