I’ve actually just begun reading A Place of Greater Safety myself, though I’m only about a quarter of a way through the book, so I don’t have a complete notion of how exactly Mantel has portrayed the character of Philippe d’Orléans aka Philippe Égalité and the extent to which he was single-mindedly motivated by ambition. The extent to which the Duke’s actions during the Revolution were motivated by idealism or personal advancement is by no means a settled question, and unfortunately a lot of the scholarship on this question is not going to be accessible unless you have a working knowledge of French, nor will be quite a few of the primary sources that discuss Philippe and his actions during this time. However, there is a good deal of evidence to help address your question, and I can give you my personal analysis on the issue you present here.

To start out, I do not think it mutually exclusive that Philippe would hold more liberal ideals regarding the governing of France, and at the same time possess a desire to see himself crowned king. Think of it this way, the statement: “France should move away from absolutism to adopt a constitutional monarchy, and I am the best person suited to fill the role of a constitutional monarch” is not at all contradictory. A constitutional monarchy is not the same as a republic, of course, and I am very skeptical that Philippe was a “genuine republican” in the strict understanding of the term. The French Revolution is a very convoluted and hairy subject, but the general consensus is that the shift from an absolute monarchy to a full-on republic was by no means a foregone conclusion, nor was it likely to have been the primary goal of most of the politically active revolutionaries (a broad category that I use here for simplicity and convenience) prior to Louis XVI’s flight to Varennes or even right up to the eve of his execution. Robespierre himself initially advocated a much more moderate position of reform.

There is also a good deal of evidence that Philippe was not simply jumping on the revolutionary bandwagon in 1789 when it became obvious that big things were in motion. In the run-up to the État-généraux, the Duke was already known for his liberal views, and he had made available for some years his home in Paris, the Palais Royal, for public debates and popular entertainment, and he had even gone so far as to establish cafés and restaurants under its arches. In the years leading to 1789 it had already become a favorite spot for speeches and political discussions. There is also evidence that Philippe was a big fan of Enlightenment philosophy, notably Rousseau, as well as an Anglophile who looked admiringly to the British constitutional monarchy as a model. And his interest in Enlightenment ideas would not have been a striking exception among 18th century nobility. There’s a laundry list of “enlightened despots” (Frederick II, Maria Theresa, Catherine the Great, etc.) who pursued reforms influenced by the thinkers of the time, so long as those reforms didn’t interfere too much with their sovereignty or the established social order. This evidence seems to be a strike against the notion that Philippe was exclusively motivated by a personal ambition for the crown, whether or not he thought it would’ve been a nice prize or that he was the best person for the job.

This is not to say that Philippe didn’t take actions that were likely to have been a bit more cynically motivated to improve his personal position at any given moment during the rapidly evolving situation in France at the time. You have to imagine that more than a few eyes rolled when he changed his name to “Philip Equality,” I mean come on. Imagine if a US presidential candidate legally changed their last name to “Universalhealthcare” (or, to use a real example, a mayoral candidate created a political party called “the Rent is Too Damn High Party”). This wasn’t the only case in which he made a dramatic political performance that may ultimately have been motivated by thoughts of his audience rather than his principles. When the time came to vote on the execution of the king, Philippe stoically voted in favor of putting his first cousin to death, and even Robespierre thought it was a bit weird, writing: “Égalité was perhaps the only member [of the Convention] who could have recused himself.”

Ultimately, Philippe d’Orléans was a flashy personality with eclectic interests and a whole lot of money. It seems likely that he had a genuine interest in the political reformation of France, and as the wealthiest man in the country, he had the ability to give things a nudge in that direction, though it’s highly doubtful that in his public political activities of the mid 1780s he had anything like an organized, calculated plot to foment revolution in order to seize the throne for himself. But perhaps, as is so often the case with the rich and powerful, Philippe regarded politics rather too much like a big game, and in his efforts to ride the currents of the maelstrom he became one of its victims. How cynical or idealistic was Philipe Égalité, Duc d’Orléans? French historian Jules Michelet summarizes it thusly: “In the end, he got what he wanted above all: he saved his money and lost only his head.”

Interestingly, I have found a Guardian article by Hilary Mantel, which seems to offer a more nuanced view of Duke’s life and role in the Revolution than his character is portrayed in the book (I’m basing this perception on what you wrote in your question - as I said, I haven’t actually finished reading the novel yet). So maybe as you suggest his portrayal in A Place of Greater Safety is more of a conscious character choice on the part of Mantel.

If you’re interested in reading a good overall narrative of the Revolution, I would recommend The French Revolution: From Enlightenment to Tyranny (2016), by Ian Davidson. In my opinion, it’s a great overview of the subject, and Davidson has intentionally structured his writing to give the reader a sense of the actual pacing of events as they happened in a really novel way. The two direct quotations that I used in my answer above come from this book.