>Why do you suppose China hasn’t already prevailed?
Peaceful reunification still priority, except one remark that force is always an option by Xi a few years ago, there hasn't been any PRC/Xi departure from this position in 8 years of his tenure. Notion that PRC wants to invade TW in near/medium timelines is manufactured propaganda.
But in terms of actual military capability: taking TW is simple but deterring US is hard.
Even ardent "porcupine" advocates from US think tank / blobs have concluded by now no amount of asymmetric porcupining will meaningfully deter PRC but is only useful to buy time for US response. SO PRC focus is to (and has been since modernization started) mitigate US response, while simultaneously making defeating TW simpler. Note simpler does not mean easy, merely all the force balance calculations of PRC taking unsupported TW has been forgone for years by PRC planners. Now US planners in last few years as well. The TLDR is PLA reforms/modernization is aiming to challenge US military hegemony, it's going after the big fish because everything smaller will be manageable by then.
And there's likely no involvement especially by SK, Vietnam (or rest of ASEAN). Likely not even Japan or Australia, US most ardent allies who has so far done nothing but theatre that doesn't credibly move posture towards supporting TW defense.