So there’s two parts to this theory, one pertaining to the story itself and the second about its actual structure.

The first part, concerning the story itself is that Tsukoyomi is Ai’s mother

She wants the film to succeed as her own personal way of atoning for how she treated her daughter in her previous life. Why we don’t know yet but I think that there are 4 people who got reincarnated total, each of whom represent a different path.

Ruby- Her new life as Ruby IS who she is, for all intents and purposes Sarina has become secondary and Ruby is her “real” life now. She represents completely embracing the new life and letting go of the old one

Aqua - Aqua is both Aqua and Goro, he’s an imperfect combination of both of his lives, where instead of being in sync his past life haunts him and weighs him down and holds his current life back.

Tsukoyomi- If I’m right, then she’s the opposite of her granddaughter Ruby. Instead of embracing her new life, she just sees it as a means to extend and atone for her part life. Her past completely defines her, and in her eyes her new life exists only to further her goals from or fix her mistakes from her last life.

Hikaru- The opposite of Aqua, Hikaru is a perfect combination where he uses his expertise from his past life to further his own goals in his current life, something happened to him in his previous life and he uses that experience in combination with the experiences from his current life to further his own goals.

Of course I could be all wrong about Tsukoyomi being AI’s mother but I think in that case she’d have to be somebody who had ties with Hikaru either in his current or previous life, who wants to see him stopped (potentially even for his own sake, maybe he’s basically an Aqua gone crazy who didn’t stop after his own version Hikaru, potentially because he lost his Ai as well as his Ruby, who in this case might be Tsukoyomi). In that case while I may have gotten her identity wrong, I still think she represents letting your past life completely define you.

The second part of my theory is that the manga is roughly halfway done, or a little bit more than that.

If you’ve read any of Akasaka previous works, instant Bullet or Kaguya Sama, then you’ll know how Akasaka likes to take his time to write his endings. (I know that ib was cancelled, but you can still feel in the final few chapters that Aka definitely wanted to try and make the most of the time he had with the finale) This is especially prevalent in Kaguya Sama, where the climactic conflict between the Shinomiyas and the Shijos, resolved the main conflict that the manga had set up, but the manga kept going for another 40 chapters after that to properly conclude.

In the same way I think that Akasaka is using the movie arc to set up that final conflict, and the arc where the movie has actually been released will be where we get the conflict between the Hoshino siblings and Hikaru, which will serve as the “equivalent” to the Shinomiya and Shijo war in Kaguya. I’d say we’ve got 15 chapters left of filming, 30 for the actual conflict and resolution - including the release of The 15 Year Lie, Hikarus response using his company, the confrontation between the siblings and Hikaru, Hikarus backstory will probably take 2-3 chapters, another 2-3 for Tsukoyomis and Ai the fallout and other scheming that Aquas up to, if people are going to die, this will be the arc it happens. It’ll probably also serve as the emotional climax for Aquas arc.

After that I imagine we’ll probably get another 25ish chapter to properly conclude the story including an epilogue with the final few chapter leading up to emotional peak of Rubys journey with her Dome Concert.

All of that is roughly 70 chapters bringing us up to chapter 200, potentially toss in anywhere from 10-20 more chapters for any unforeseen mini arcs, extended flashbacks or explanations, or just because of longer arcs and you’ll get ~220 chapter maximum, at an average of 40 chapters per year that’s roughly the manga ending December 2025, which is around the same time Renai Daikou would be entering its 100th chapter or so, which doesn’t really mean anything overall but it’s just a fun little coincidence.