I was thinking about this subject the other day, but would a few thousand Micronesians be enough to organise a Ryukyu-like kingdom? Some estimates have small islands like Tinian (100 km2) having tens of thousands of people. If we look at the Bonin Islands and the main islands there of Chichijima and Hahajima, you could have a few thousand Micronesians.
I'd imagine the nucleus of this kingdom being Chichijima and Hahajima (about 43 km2 combined), as well as Mukojima adding another 5 km2, allowing a compact kingdom. South of it, you have the Volcano Islands centered around Iwo Jima and Kita Iwo Jima allowing a transition to a more typical Micronesian culture. North of it, you have Torishima allowing an area of contact between the Izu Islands--and Japan as a whole--and this extension of Micronesian culture. Japonic influences thus have a path to influence these islands, and Japanese sailors have a reason to sail into the tropical Pacific. I have no doubt that a successful Micronesian settlement in these islands involves contact with Japan via the Izu Islands.
Chichijima and Hahajima are close enough to each other that they could easily fall under the influence of a single ruler, much as Ryukyu was unified. Mukojima likewise could have a few hundred people who might be unified under those two islands. I doubt they'd be able to influence Iwo Jima and its surrounding islands, but would carry on relations with them. I could imagine that these Micronesians would be extensively influenced by Japan and by proxy China. Their language would be written with Chinese characters and have many Chinese and Japanese loanwords, while their culture would no doubt have those East Asian influences. Like the small and remote island of Ulleungdo was once an independent kingdom before Korea annexed it, this Micronesian state would be similar as a remote state influenced by the regional power.
A few thousand Micronesians on these islands, in contact with the Izu Islands, presents an interesting situation. Unlike the Ryukyus, China will have little influence there compared to Japan. Certainly the islands wouldn't be called "Bonin" (dialectual for "uninhabited"). I wonder if their Micronesian language survive into the modern era, or if they would be Japonicised sooner or later and abandon their language in favour of a unique dialect of Japanese (most akin to that of Izu Islands perhaps).