This is quite the contrary. Natsumi Schwarz is Subaru’s way of erasing his identity, the part of himself he feels has failed. He hides this from the people he cares about, example, Emilia. Natsumi is a tool to be useful and the moment he is again useful as a man he discards it.
Daily reminder that Subaru is a queer coded character. If you think that his crossdressing was just for mission's purposes then you basically LARPing the series or denying a core part of his especially how that conflict drives his inner turmoil in Arc3/his overall identity crisis