With Russia having gutted their defenses in the Far East to support the war of conquest in Ukraine I doubt that China will attack Taiwan.
Too many downsides involved in attacking Taiwan especially given the Japanese promise to defend Taiwan. The Aussies are also saying they'll step in and the Aussie submarine forces are no fucking joke.
That said, with Siberia having been stolen from China 150 years ago this is their golden opportunity to take it back while Russia is dreadfully weak. Plus there's no downside from the West should China retake territory that arguably belongs to them anyway. No one in the West is going to sanction China for kicking Putin in the balls.
I disagree about China not attacking Taiwan. Their only viable option would be to attempt to capitalize on a distracted US in an actual US versus Russia conflict. The longer they wait the less likely they will be able to take it. First, Taiwan, Japan, South Korea, Australia, and others in the Asia-Pacific region are, as you said, prepared to defend against a Chinese invasion. While China is continuing to increase its naval and air capabilities it is not doing so in a vacuum ; Japan is expanding its naval capabilities, as is Australia, and South Korea is making great strides in its military technology - in fact I believe I saw somewhere recently that South Korea implied that it was only a short bit away from being able to field nuclear weapons if it chose to. Taiwan is not neglecting its defenses either, and the US continues to supply them with technology, munitions, and most importantly intelligence. Given the significant advantage determined defenders have against any kind of attack I am skeptical of China's ability to expand both rapidly and large enough to overcome this advantage even without direct US intervention.
China does not have the luxury of time for another reason : politics. Taiwan and its people are becoming more and more resolute in their Taiwanese identity. With China continuing to take a hard line, antagonistic political stance with the cross-strait issue they further encourage this solidarity amongst the people of Taiwan. It is very rapidly coming to a point where no one left in Taiwan even remembers what being "China" was like. If China had taken a more delicate approach of detente between the two countries and instead attempted to welcome Taiwan more openly then perhaps Taiwan's people would be more willing to one day agree to a peaceful reunification with China. As it stands now, with China continually denouncing any attempts by any other nation to legitimize Taiwan and throwing a hissy fit any time someone mentions Taiwan as being its own independent entity, I don't see a diplomatic approach ever working.
Finally, China is facing an enormous demographic crisis. Its one-child policy accomplished its goal of slowing their rampant population growth but it also created a bottleneck which is rapidly coming to a head for them. Pretty soon their large population will entirely age out of working age - let alone fighting age - and become a significant fiscal burden on their economy. Combined with the ever-present brain drain in totalitarian regimes China does not have the demographic - and thus economic - strength to wait much longer to take "back" ( I use quotes because the communists never controlled Taiwan in the first place ) Taiwan. If the US and Russia ever do fight then China
must use that as an opportunity to attack Taiwan or else they will lose any chance at doing so in any foreseeable future.
At this point I'd say that the territory belongs to Russia. It's far beyond living memory, and there is a striking level of hypocrisy in allowing China to take land from Russia but refusing to allow Russia to take land from Ukraine ( or Georgia, as Obama should have done originally ). The people in that "Chinese" part of Siberia may not see themselves as Russian but I doubt they see themselves as Chinese either. While it is possible China could attempt to capitalize on Russia's weakness I don't think they'd have much to gain from it, and they would certainly be politically ostracized if they attempted to do so. Taking this Siberian territory would probably be easy enough for China to do but the long-term cost of it is too great. They would lose access to Russia's vast gas and oil supplies and then they would have to attempt to Sinofy an already heavily Russified people. At best this process is simply a slow and cumbersome process to teach the people Chinese and change their societal identity to being that of "China" which requires large amounts of resources to accomplish. At worst they have to deal with an armed insurgency of anti-Chinese partisans - who will absolutely be supplied by both Russia and the West - who will do everything possible to resist becoming part of China. The Chinese may be arrogant but I don't think they are stupid either ; taking territory you once held 150 years ago is not a reasonable thing to attempt in the best of circumstances, let alone one where they would face significant censure from the international community.
