Okay, much of this has to rely on a lot of speculative/semi-conspiracy theories of the time (but what "what-if" scenario doesn't).
The most challenging parameters you have set are:
- Taiwan is de-facto independent, but de-jure part of China, just like today.
- Taiwan has sufficiently amicable relations with China to import military equipment from China, despite having an independent government.
These are very conflicting requirements but I think there's a way to work this out.
First of all, we have to understand the relationship between KMT and CCP. While Dr. Sun Yat-Sen was still alive, the CCP was a recognized political party within ROC. Many Chinese elites of those times studied both philosophies and many of them were colleagues and have close personal ties. For a lot of them, there was no reason at the time to believe "Communism" was inherently evil, bad, or that it wouldn't work. Both the KMT and CCP were broadly committed bring power to the people and rebuilding China after the successful overthrow of Qing dynasty in 1911.
Just because the Qing Empire was toppled doesn't solve a lot of inherent issues of China overnight. Local warlords, the norm of various corruption in society, these don't go away. KMT inherited a lot of these problems and the wealthy class remain wealthy, so it was very normal that Marxist ideals had its appeal. It was only after Sun Yat-sen died (1925) that Chiang Kai-Shek tried to purge the communists (1927).
Many of Chiang's closest advisors and generals, were sympathetic to the CCP despite remaining loyal to KMT. This includes Chang Hsüeh-liang who kidnaped Chiang to force a ceasefire with CCP and united front against Japan, and the war hero Sun Li-jen
Sun Li-jen was viewed very positively by the US and it was rumored that he would be a prime candidate for being installed as a puppet president. He's idealistic but naive. As Chiang Kai-shek once said of him, Sun Li-jen is good at fighting battles, but he's no good at winning wars. He's not much of a politician.and probably not great at managing international relations.
First, Chiang Kai-shek needs to die, and most probably taking place after Chiang Kai-shek walked away from the UN in 1971 refusing US/UK's offer of being recognized as a new country (i.e. puppet state). His death would likely be due a successful CIA-assassination attempt. After that, the US political machine drums up Sun Li-jen's war hero status and install him as president of Taiwan.
However, this alone is not enough. While Sun Li-jen never sees the CCP as the enemy, he simply lacks the political saavy. However, what he would likely do among his first acts as president, would be to pardon Chang Hsüeh-liang from house arrest, who would have the seniority and history to be his advisor. Both Sun Li-jen (b. 1900, d. 1990) and Chiang Hsüeh-liang (b. 1901, d. 2001) would seek to improve relations with the CCP, upholding the pan-Chinese identity. Chang would probably have the political backbone and wit to tell the US to backoff while simultaneously keep the naive Sun safe from assination when the US realized their puppet is not much of a puppet.
Under Sun Li-jen, travel bans and other cross-straight restrictions would probably be relaxed a lot sooner than 1987, possibly not long after 1976 after the cultural revolution ended. This would significantly improve morale of the Waishengren who retreated/immigrated along with the KMT army from the mainland, families reunited, etc. etc. Since China hasn't build up its economy and educated class, but with improved relations with Taiwan, Taiwan would take the lead in a lot of initiatives. Today's economic environment would be VERY different as most of the Mainland's private enterprises would probably actually be owned by Taiwan, and Taiwain maintains its status as the envy of the Mainland (which today is only an outdated story Taiwanese tell themselves).
However, if Sun Li-jen became president, that does not bode well for the Benshengren. Although Sun Li-jen has objected to Chiang being authoritarian, I believe Sun is more likely to uphold a pan-Chinese identity that see's Mainlanders, Waishengren, Benshengren (Taiwanese colonial-nativists who migrated from China 300 years ago) all as one people, while Benshengren a promoting a "Taiwanese" identity. If Sun Li-jen became president, I'm not sure Chiang Ching-kuo would succeed him. If Chiang Ching Kuo did not become president, many of his policies intended to be more inclusive and support Benshrengren (despite their blames and hatred of KMT) would not come to pass, and Li Teng-hui who truly catapulted Benshengren representation in politics (and paved way for the first DPP president to get elected) would not have became president. Taiwan in this alternate universe would actually be genuinely "Pro-China", though it may not necessarily have the same negative connotations today as Taiwan would take economic lead in this world.
With all that setup, private enterprises would be dominated by Taiwan, but China would have the industrial base to supply Taiwan with military equipment, which they will need to use as earlier refusal to be puppeted by the US means the US would not supply Taiwan with weapons.