I don't watch or read a lot of idol media besides Perfect Blue (a similar concept with an idol suffering from self identity due to the system) but I'm gonna lob a free throw and predict the ending since I'm picking up the vibe it might go down a similar route that To Be Hero X did.
Sanghyeon / Yoil officially debuts, probably does a lot of press photos, modeling shoots (PR stuff) and garnishes a lot of attention. This fuels Jeong's contempt for Yoil which has been building up for the last few chapters and she discovers Yoil’s true identity. Yoil gets exposed for leading a double life / being a man (from either Jeong, a stalker like in Ruha’s arc, or someone else), people are deceived and potentially angry especially since people absolutely love idol's personal lives for some reason. Sanghyeon has to give up the mantle of Yoil. But the identity exposure doesn't crumble Sanghyeon as a person because we've seen time and time again he truly pushes himself to the brink to achieve his dreams (getting nose bleeds by pushing both of his bodies too far, embarrassing himself by losing his masculine features). It forces Sanghyeon to take front stage because he literally has no other option left. Thematically it fits as well, because Sanghyeon’s characteristic is perseverance and the only way to demonstrate that is by breaking the character's comfortability and stability (we dub this, the Spider-Man effect). And instead of trying to rebuild Yoil or apologize for existing, he just keeps doing what he’s always done. Working, pushing, grinding, even if it means embarrassing himself or trying to win an uphill battle. He’s already proven he’ll go to the brink for his goals.
A side note as well;
The reason I believe he won't try to reclaim Yoil as his persona is based purely on his method of using her in the first place. It was never about a psychological or emotional aspect, rather a means to justify the ends. If he had become an attractive man, the story’s plot points would play out the exact same. His transformation was never truly integral to his goals. It was only integral towards his internal change. A catalyst rather than a closing remark (if the plot point of being able to transform into your past selves wasn't introduced, Sanghyeon would still be chasing his dream).
Towards the end of the story people finally see him. Without Yoil’s face shielding him, the raw effort becomes visible. The industry, fans, even the people who were angry, they can’t deny the talent and drive that was there the whole time. The persona and idol the public knew may have been “a false pretense”, but the work behind it never was. Yoil essentially “dies” or retires (and the impact of her death, figuratively and physically, will hit us hard since we've seen a majority of the chapters with Sanghyeon as Yoil), but Sanghyeon doesn't. He keeps moving forward and whether or not the world decides to move with him, he's pushing no matter what. I truly believe, even if I get this totally wrong, that Sanghyeon’s one defining trait and characteristic isn't Yoil, but his persistence and ability to change into a better person. Where the conclusion and theme of the entire story is not transformation but continuation through self assertion, where Sanghyeon isn't being changed or hardwired to be someone else but finally being seen as him. A very good ending in my thinking is Sanghyeon accepting that he cannot hide forever behind a more marketable version of himself. Not because being Yoil was worthless (literally the most integral part of the entire story is Yoil), but because eventually he has to exist as himself and accept whatever consequences come with that. It would put a nice little bow towards an overarching message on how idols change themselves and lose essential parts that make them them (and how that's kind of a bad thing).
Also looking back on Geukjang’s (author) other really really good webtoon A Savior's Time, where the theme and takeaway lesson is that escapism isn't a solution to your problems, I feel like this story is gearing towards a very similar status quo. We've seen Sanghyeon adapt and become Yoil in many ways, but Yoil is still and will forever be seen as a form of escape from the mistakes that Sanghyeon has made in the past. To truly come to terms with himself and his mistakes, escaping isn't the true answer. Although, knowing from Geukjang, the ending may be really bittersweet. Where the world doesn't actually accept Sanghyeon as himself and he never truly achieves his dream but still persists despite it all.
Oh and Lee Ruha is endgame.