Speaking here as a historian, not in my role as writer.
I don't know how the Thaksin/Ramkhamhaeng thing came about - Thaksin's family is also Chinese-Thai, too - Ramkhamhaeng would have physically looked quite different (and also quite different from the often half-European, half-Thai actors that play historical heroes in present-day Thai films).
Each prospective leader - Ramkhamhaeng, Narai, Rama V, Rama IX - would have a distinct focus and problems associated.
Ramkhamhaeng's historical sources are sketchy, and large portions of his story have their historical veracity contested. He would hail from a semi-mythic time period just after the Khmer empire started to collapse and new city-states appeared on the frontier. Imagine King Arthur.
Narai I agree is an interesting figure. Ayutthaya was a thriving metropolis during his time, and had a large Japanese population, a Persian delegation, a Greek chancellor - in short, it was a real, independent, thriving Southeast Asian metropolis. Out of a partial fear of this cosmopolitanism, he was overthrown, but it remains a great choice for a medieval/Renaissance Siam/Ayutthaya. Imagine a strange king, one who embraced a new possibility (but was ousted for it) - hmmm I can't think of a good example.
What Sukrit is intimating about Naresuan, I agree with. He was a militaristic king who led a series of wars against Burma, but most significantly he is the poster hero for a new kind of media-led militarism in Thai society. A series of big-budget historical movies had Naresuan as the hero, and he becomes the handsome face of Thai militant nationalism. The question of whether or not he fought the Burmese king on elephant-back is likely a historical myth, but Thai historian Sulak Sivaraksa was charged with a potentially decades-long criminal charge for suggesting that this duel did not take place and urging people to not pay attention to propaganda, so I'm certainly not saying that right here (charges were dropped). Imagine Richard the Lionheart.
Rama V is a fantastic choice, I think. I understand Sukrit's hesitation in voicing his avatar, but he really transformed Siam from a shaky buffer state into an independent, industrializing state - not a free one, necessarily, as he modeled himself more on European absolute monarchs than either past Siamese monarchs or on constitutional monarchs, but really one of the architects of modern-day Siam/Thailand. Imagine Victoria.
Rama IX is not going to happen, but for a late atomic-age leader, this would be the choice. The American-born king moved the monarchy from being a symbolic anti-Communist figurehead to a real media-fueled source of a particular kind of nationalism. Way, way, way too recent and too controversial. I wouldn't even suggest him. I'm not going to suggest a comparison.
Looking beyond kings, though, there are other interesting choices. Pridi, for instance, was the democratically-minded statesman who sought to make a more open Thailand in between the aspirations of absolute monarchs on one side and fascist-adjacent military leaders on the other. He remains well-loved in Thai academic communities. I'd love to see a Pridi-led Thailand, but it's perhaps too niche a choice.
Phibul would be Pridi's frenemy - the nationalist and fascist-adjacent military leader who spurred the temporary abdication of the monarch and a new, nationalist Thailand (as opposed to the prior absolute monarchical Siam). Again, an architect of a possible Thailand, whose legacy is currently being hotly contested (and erased) as we speak. Never going to make it - "fascist" and "hotly contested" are two big red flags.
Sarit would define Cold War Thailand - a military dictator and US ally, with a long, long list of negatives to his name, but he did shape the country into what it would later become - restoring the monarchy and firmly putting Thailand in the US side of the Cold War - through violence, when he had to. Never going to make it. Imagine Suharto.
While I like Pridi best of all, it's not the popular choice. And various other reasons (controversy, an over-mythologization) put a bit of a damper on all except Narai and, I'd argue, a well-handled Rama V.