Character(The Rebels)
Dioscuri (Progression type) (3 required Major Curses)
Crowning Curse: The Curse Of Hubris (+6 RV)
Major Curses: Affliction of Vigor, Doom of the Coward, Affliction of the Figment
Primary Remittances: Inheritor (Sable and Serring Barr) and Twice-Great (Time Skein [Fractal, Wing] and Crown and Thorns [Mind, Corsage {Willpower}]
Lesser Remittances: Binary System, To Shatter Heaven, Rebuilt (Recover [Sun Yijun's mental illness] and Redress [The Souls not realizing humanity is sapient])
So, the most obvious reason this build is questionable is that is includes To Shatter Heaven. I'll leave that for last.
The original reason I thought of this plan is explained by the spoiler title. I didn't trust myself to make a Persona build, and the Rebels were the only other option that could become a Progression type without taking the Geass of Indenture.
That being said, I also wanted them to have both Pandyssian Blood and Corsage (Willpower). So that locked in Inheritor, Twice-Great, Binary System, and Curse of Hubris (as the only Crowning Curse that can be outscaled in any semi-permanent fashion).
It was looking at those requirements that made me select the rest of my build. I'm already using Inheritor on one of the premade options, so the second should be one of the two remaining ones. They're already feeding the Corsage off the Mind to take advantage of Songs having at least a few decades of forgettable memories from their 1200+ years of life, and whatever limited recursive loop manages to form from Pandyssian Blood, and maybe some things from the past lives, so Mentallics, after some Progression, should allow them to copy other people's memories without hurting them, and give it more food that way, prioritizing Barr. Then, with one magic system slot left open, and already using Inheritor, they went with Time Skein and the Fractal Perception to maybe get more out of that, either in more lives, or more thorough connections to the other universes. Wing was just the specialty that seemed most useful alone.
I then needed to decide on their three Major Curses, and consider what to take to allow them to survive having a Crowning Curse. They can absolutely outscale their home universe to the point that Hubris is no longer dangerous, but they won't start there, and they also won't be avoiding conflict that could ruin them. ...Avoiding Conflict... Doom of the Coward? But that's about fear. How could someone with the Curse of Hubris be too scared to do anything? Well, these two are the nice one. Really, look at their goals and their Swords upgrade. And their powers are heavily focused on the mental. They're not scared for themselves, they're scared they'll erase anyone they get in direct conflict with and replace them with puppets. Even long before they have that capability.
So they'll go for the light touch, using history and psychology (and eventually proper psychohistory) to identify the right places to push, and Theft plus Guerrilla Warfare to get into secure areas without directly attacking anyone to mess with paperwork, and stick to Read-only with their Mentallics, still boosting their social arguments but not to cause mind control. After all, they want to change the opinion of an entire interstellar society, and that needs to be done by guidance. Doing so by force either fails, or destroys the society and replaces it with something completely different in the process of succeeding.
And that means no-one will learn about their powers, since they don't use them blatantly. So Affliction of the Figment isn't a problem, because no one will ever know to try to resist them. And between Songs being a Soul, already being partially illusory, and their connection to The Void, pyschic fields, and alternate dimensions, they should be entirely ready to transition to being a purely psychic entity living between worlds but able to manifest a physical avatar when necessary by the time Vigor comes due.
...I think. Between their three inherent abilities and their Fractal Energy (and specific focus on perceiving it's stickiness), this should be possible. But they may not be conceptually able to combine that well. And that's why I took To Shatter Heaven. And now that we're on the topic, it really should be available, at least by what's written down. What Beloved actually says is "Pick any Lesser Remittance offered in a past AST quest or an Omake quest, or any person or item from a past Rihaku quest at any point in their existence." So, while your statement I quoted says that you didn't intend for this, every Upgrade Less Remittance is completely valid, as written. Thus, why the build is questionable.
...And if I'm being questionable anyway, let's spend that last Remittance on Rebuilt to Redress the main issue of the setting, where the Souls in general made a mistake and thought humanity wasn't sapient. It won't change the setting, but any extra personality from Redress should have a synergy with both Inheritor and Twin Souled, and the memories of what that alternate society was like should help in coming up with a plan to make their current society come to the same realization. It also let's them fix Sun's mental illness, which is more of a nice bonus than crucial after they learned how to work together, but is still nice.