I dunno. I almost always dislike doujinshi that make Rin either trans or futa. It's not because I have an aversion to either thing, per se. It's because the entire point of her development of character in the show was that she was uncomfortable showing her feminine side, and her friends had to really fight to show her that she not only was just as cute a girl as the rest of them, but that she had her own unique type of femininity and adorable girlishness that made her stand out with that Rin brand of sweetness. Futa type stuff makes me kinda sad because it feels like it undoes all that struggle and eventual acceptance to be how she wants to be. (Yes yes I know it's an anime and fan art/stories so anything goes, and that's okay. It's just my personal feelings on the matter for this particular character.)
I agree with that, and on top of that as a trans woman myself Rin never, ever feels like she's trans in the series. If anyone comes off that way, it's the tentative yet passionate Hanayo. Rin, for her part, comes off as a tomboy who harbours a desire to explore a part of herself she never has before, but not in a "trans" way if that makes any sense.
The bigger sin of course is that taking the least feminine character, slapping a dick on her and calling it done, is pretty much the laziest option from a writing perspective.