[Vespers War]

I wonder if Taiwan isn't more useful to China, or at least some factions within China, as a semi-external threat than it would be as a fully integrated territory with high levels of unrest.

While China has land border disputes that justify spending on the PLA, Taiwan's presence as (from the PRC's perspective) a breakaway province justifies spending on the PLAN in different areas than the potential threat from the United States. There's much less need for landing ships and amphibious warfare equipment if Taiwan's not a potential target, but making it an actual target would remove the utility of those forces.

An invasion also runs the same threat of weakening Chinese soft power that Russia's running into with reactions to its invasion of Ukraine. Once you start playing hardball and invading other nations, it's harder to convince countries where you have major projects that you won't do the same thing to them if they get cold feet in the future. I don't have a great grasp on how China figures its strategic calculus, but in my view they have more to lose on the global scale than they would gain from invading Taiwan, regardless of the outcome of the invasion.