So, I stumbled upon this book while browsing Kindle Unlimited.
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It's a typical AGP fantasy. Sarah is revealed to be a transbian, Chris likes to wear his sisters underwear and clothes and of course gets off on it. Once Sarah is revealed to be a troon (got put on blockers and HRT and passes so well that no one knew, except for some bullies), Chris's girlfriend somehow know he's a troon too, and invites him over and let's him put on her underwear and clothes, because it's so obvious he's actually a girl! That's about as far as I got into it, because it's rather boring and not well written.
I found out the author Morgan Snow is a user on reddit:
https://www.reddit.com/user/thesnowgirl147/
He's a teacher and lives in Colorado, and is eager to groom his students
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Of course he posts on lesbian subreddits, except he tries to hide the fact that he's trans and pretends to be a cis lesbian.
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Apparently he has another reddit account, where he's more open about his troon status and troon topics, but I didn't feel like trying to find it, but if another kiwi is up for the task, go for it.