US AD have failed repeatedly against even the third rate 1980s SCUD. They say they corrected the clock issue from the original Patriot that achieved a whopping <10% success rate against SCUDs but Iran doesn't use SCUDs.
US AD has no chance intercepting a third rate (by China's PLARF standards) DF-15 let alone a real HGV like DF-17.
You're also forgetting China would perform saturation attacks against US. In this regard, it is worth shooting less capable and older missiles not in an attempt to deplete AD (or the actual target if not the AD site) but to overwhelm its radar tracking and operators. For every missile Iran is capable of throwing, China is capable of throwing 100. It's fair to say China outproduces just about anything you care to think of compared to Iran by more than a magnitude of 2. Let's take something like cars or ships over 500 tonnes. China has build more than 100 times that of Iran with ease. For most products it's a magnitude of 3 or 4 at least. Google what magnitude actually means if you don't know and if you think Iran has 500 ballistic missiles stockpiled, you best believe China would have 50,000 if it is marching to war against the US. Difference is USA has no chance of air superiority even within the first island chain, let along over China. The factories will be running 24/7 and if they are hit, American factories aren't intact either. DF-5, FOBS, DF-31, DF-41, DF-45/51, CJ-1000, JL-1 ALBM, JL-2 SLBM, JL-3 SLBM, YJ-20, YJ-19, YJ-17 will ensure no American MIC is left intact.
US stands zero chance in this region against China if it's getting its ships obliterated with ease by Iran within just 18 hours of war. Iran has managed to take out several US airbases protected by multilayered integrated American air defences. Take out a US warship. Take out at least two important over the horizon radar units. All of these protected by integrated air defences.
USA is performing worse than Russia. For all the shit NAFOids gave Russia, looks like Westoids bend harder during actual war. They fully clowned themselves and worst part is they don't even realise or notice just how much Iran is embarrassing them. Honestly could never have expected so many US airbases to be hit so easily.