I would submit you don't know a damn thing what the Chinese leadership is preparing for or not, so that is worthless speculation on your part. I suspect otherwise, based on obvious evidence seen daily, but have no interest in researching it for you.
The fact is: China is currently actively and openly preparing for war with Taiwan. They have a 4+ year count down to war time frame which they have announced internally. This has not been publicized or discussed by our gov't for public consumption here as it would give us less wiggle room diplomatically. Do you not wonder what the US will do when this inevitably occurs?
Shockingly: most other countries in SE Asia currently believe that China would currently win a war with the US. This will become more pronounced as they increase their capabilities via the transfer of dollars from consumers.
The Chinese should be our allies, yet our hegemony and various poor forigon policy blunders scares them. I suspect that a conflict over resources will sadly occur eventually, as it did with Japan during WW2.
How would China taking Taiwan pose a national security threat to the US?
Sure, it would set a bad tone on the international stage. It could be an opening salvo to Chinese hegemony in the east. But, what effect does it really have on us here?