I like to think that I'm fairly knowledgeable on gender/sexuality (being part of LGBTQ+ myself) while still being able to look at these issues objectively without being caught up in "sensational" ideas like: having pronouns/labels for every possible variation of gender/sexuality, or having to heavily modify your body just to be comfortable in your own skin.
That said, I really can't tell for sure what Oshimi's trying to deal with (in his own life, not his works). All of his "after-words" in this series and others just contain a whole lot of nothing. I'm not sure if it's because he has too many issues that are interwoven, making them too complex to properly explain/express; or if he has some simple issue that he just really really sucks at explaining/expressing and is overcomplicating it. Still, I think it's the latter, so I'll try to give an (somewhat surface level) interpretation of his mentality anyways:
- It all started because he became disgusted by his own sexual desires due to societal norms telling him to be. And he happens to also dislike how he looks or looked due to puberty. Both of these combined caused him to want to be clean/pure/beautiful.
- As such, he wants to be a female, because he associates those ideals with the idea of "femininity". From what I can tell, he thinks this because he viewed women as completely holy/perfect/clean.
- But then again, this contrasts with how realistically girls and women have been portrayed in some of his works: they're not perfect and clean by default.
- Still, even if he recognizes that, he still places "femininity" on a pedestal due to how unattainable it is to him (both physically and mentally). He wants to actually be a woman because he thinks there's some hidden secret he will never know about due to having been born a man (spoilers: there isn't). And if he had been born a woman, everything will just click and he'll be happy (which is just way too idealistic).
- But besides all of the above ideas, something he strongly believes is that societal norms can be really toxic. He also wishes for people to treat individuals as the complex individuals they are, rather than treat them as neatly-wrapped packages with labels perfectly describing what they are. These are expressed quite strongly in this manga. (These are just valid ideas and not an inner turmoil like the ones above)
To summarize: the societal norms regarding being a man have made Oshimi deeply unhappy from a young age so he wants to not be "a man", but he can't (for reasons he can’t articulate).
My answer to his dilemmas (and it’s possible this is too narrow-minded) is: he’s overcomplicating things. He just needs to recognize that there's nothing inherently wrong with sexual desires; it's not inherently better or worse to be a woman; and societal norms aren't always "correct". Feel free to like what you like (if it doesn't actually harm anyone) and act however you want (if it doesn't actually harm anyone). Life is really not as deep as many people make it out to be, and Oshimi is just stressing over nothing.