I love the existential questions you raise here, Saoirse. Reminds me of some classic science fiction I read as a young adult. But never with a trans theme!
Your conceit of a bland waiting room and a trusting child is a marvelous way to set off the story. What seems ordinary if a bit nerve wracking at first quickly builds in tension.
And a nice couple bits of foreshadowing are very cool on a second read!