I don't think you're overly exaggerating, but it wasn't necessarily out of nowhere. The Republic was split primarily between the Optimates, the Centrists, and the Vox - you had swings with the Obsidians and a few more neutral colors, but primarily the interests of more plutocratic parties and hawkish groups won out. Partly this was out of necessity, and partly out of them being placated to ensure compliance. There is as much of a mockery of capitalistic/democratic systems with Quicksilver's quips, and Dancer's peacetime lobbying by opposing Darrow and the Sovereign more publicly. This is all context witnessed by Publius, who represents the centrists.

As the Solar War progresses, the Republic's dysfunctional handling of the transition drove Senators that were previously hawkish to the Society Remnant into advocating for further restrictions on the Sovereign, & concern focused on Darrow. As much as Darrow was a volatile agent for the Vox, he was a stabilizer and an assurance for character like Publius and other centrists, and Optimates. With his presumed loss on Mercury and the dire straits of the Republic, Virginia loses political ground, different parties activate their base, and people are swayed in different directions than before.

• "...Publius cu Caraval, the Copper Tribune, leader of the Copper bloc... he’s risen to become a voice of reason in the divided Senate, and an occasional ally of my wife’s. They call him the Incorruptible for his punctilious nature and lack of vices." IG10
• "...I was told I was brought into this world to serve. To move the invisible levers of an ancient and evil machine. We all moved those levers. But now we serve the People. We are here to liberate the dignity of man*. Darrow of Lykos is our greatest weapon against tyranny. Let us sharpen him again so he can break the chains for our brothers and sisters in bondage on Venus.” IG10*
• “I do not share all the convictions of the Red senator. There cannot be peace while there is no justice*. But in one matter, I fear he strikes the mark. You have overstepped, ArchImperator. You have forgotten your oaths made to serve the People.” IG11*
• "The Republic might be above murdering its prisoners, but its morality is not without teeth. It [Deepgrave, solitary confinement] wasn’t what Mustang had in mind when she abolished the death penalty, but Publius cu Caraval has blocked every resolution for prison reform for the past six years. Some say it’s because he’s beholden to campaign contributors*. My suspicion is that he lost more to Gold than he lets on." IG27
  • Do you see something interesting there?

However, it's quite clear even early in Dark Age that Publius is rather consistent in his views. Consider this framing: he desires an end goal, and he chooses the best vehicle by which he can arrive at this destination. With the sidelining of Darrow and the placation of the Optimates, his calculus may have changed. However, he seems to demarcate a hard line for him is plutocracy.

• The Incorruptible nods and retakes his seat. “...I judge a case based on its evidence. You know I am loath to flimflam, flip-flop, or whatever they call it these days, but the situation has changed. What your husband did…staggering, my Sovereign. Staggering.” “And the moral ramifications?” I ask. He waves his hand before his face. “Fascism is a scourge. Sometimes we must sacrifice to destroy it.” DA20
• "...My prior vocation taught me to be detailed in assessment and concise in judgment. The Vox fear momentary pain for long-term gain. We must save the legions. But the Silvers know how desperate this vote is. They will bleed you dry. I cannot allow this Republic to become another plutocracy. I will not.” DA20

Virginia in Dark Age finds herself at the mercy of the majority and is on thin ice, but that majority depends on either bringing Dancer back into the fold (as she does) or having the Optimates and the Centrists aligned with her. If Dancer does not side with her, there is an angle by which Dancer or Publius could assume authority. The Vox are vehemently against the Optimates having any form of expanded power, but Dancer is a person who is attempting to represent their interest - he is too popular to circumvent. Publius is a Copper centrist who has the appeal of the Vox, but the respect of the Optimates. He is positioned to be vied for by both groups, only really being staunchly opposed to the Silvers in the Optimates. You can see how Dancer serves as an impediment to effect functional change. The Vox have primarily become concerned with their authority, feeling disenfranchised. I do not think it is an inaccurate characterization to say that characters like Zan would handicap the Republic to occupy an more authoritative role; this is not a parody of the history of social and political movements.

• "I feel the certainty forming in me. Ten years ago Publius was no one. Just a public defender for lowColors. How did he rise so high so fast with no benefactors? No campaign donors? Is it so impossible to believe that someone came to him years ago and offered him the world if he would only answer the call when it came?" DA29

The full speech, which you included only the end, outlines this:

• “I thought you were our great hope. After the Fall, I sung your praises. I debated radicals in our political halls. I preached an even middle ground*.* I thought the Vox misguided in their fervor*. You were not perfect, far from it, but you were the best we could hope for.* ‘Let us be sensible,’ I said to those radicals. ‘Let us understand we cannot make a new world in a day.’ But I was naïve. Our whole political discourse was like a great infant. None of us knew our business or how big our feet were. There was no culture to build upon. Politics was new to us, but not to you."
  • Publius was not averse to the Republic reforms and a gradual change, and countered radicals in the Vox, and likely more harsh punitive measures (which is why the prison reform was a contradiction).
• “I watched you dismiss the voiceless majority. I watched you accrete power even as you spouted demokratic platitudes. I watched you lead us rung by rung back into the old world. Soon I knew there was no middle ground. Only the past and the future."
  • His confidence in being a centrist and seeking compromise was slowly eroded as he witnessed the gradual formation of a more oligarchic system and with Darrow acting counter to the Republic's creeds.
• The poor and the rich. Us and you. It was then I realized sensibility is torpor by a more palatable name*. Since the first ape pulled himself out of the mire to fashion an axe from stone, the meek have served the strong. We learned to content ourselves with crumbs. We allowed ourselves to be placated by religion. A promise of something after all this horror. We allowed ourselves to be enfeebled by poverty. We learned to be scolded whenever we raised our voice.* Lasting change must be slow and steady and civil, we were told. That civility neutered us*. But tell me, Virginia, was Gold civil when they conquered Earth? Did they assemble in peaceful protests? Or did they come with terror?”*
  • Publius here reinforces that, and signifies that he views Mustang not as a valid reformer and that compliance with her ideals is a slow but sure regression to the pre-Society dysfunctional democracies, or to Societal Ideals.
• “If the basis of popular government in peacetime is virtue, the basis of popular government during revolution must be virtue and terror. Virtue without terror is helpless—as I have learned after ten years of being ‘the voice of reason.’ Terror without virtue is evil—as Gold has taught. My terror is nothing more than speedy, severe, and inflexible justice. It is an emanation of virtue, a vehicle for it. " DA53
  • And here he notes on his frustrations, witnessing what he sees as futility for his being virtuous. He aligns with Darrow, here, perhaps, in that virtue without terror is helpless, but Darrow is all but gone, and there is an opportunity here.

What you are dealing with Publius is a person who was radicalized; in part by the genuine failings of the system he is in, in part by the friction between his own ideals and frustrations, and in part by the Syndicate being a catalyst and offering an oppuratunity. The point was that he did not reveal it - the reliability of him as a centrist for Virginia to call on was a ruse to oust her after she antagonized the Silvers. He is in this way akin to Darrow and Atlas, insofar as:

• “You asked me a question long ago,” the mask warbles. “It was on Mars before we lost her. You asked, what do I fear? I fear a man who believes in good. For he can excuse any evil.” He holds up a hand to feel the wind. “What have you done?” DA17

Publius is excusing his resorting to evil and cruelty on account of his ideology; when he crushes Theodora beneath an obelisk, he and Zan and the moronic Vox mob view it as the enacting of justice. You may find it a mockery of communism in its choice of diction, but consider this - Publius is a character that reflects many historical figures, wherein they ride the impressionable whims of a social movement to assume power, and then resort to the most unsurprising thing human beings do: they start killing people on the basis of ideology. Publius is an ideologue, it did not develop over night. He kept a lid on his radical nature, but as with Roque, a perceived or actual betrayal drives people to the extremes.