this would be a bad idea if Iran was not also accommodated. You can't give new friends something and leave old friends out. Note that there's no international sanctions on Iran (only unilateral ones), and they have the capability to pay.
Yeah it won't be easy to fit Saudi in the region, but if they are openly willing to defect, then there has to be some type of roadmap readied for them.
DF-21D for Saudi does not directly threaten Iran. Iran doesn't really employ ships large enough to be targeted by them. IIRC since the 21Ds are older missiles, about the smallest things they can reliably target would be large FFGs/small DDGs like 052D or Baden Wutenburg class.
But honestly if Iran pays, it makes sense for them to get DF-21D as well. They wouldn't be able to commit much aggression using the missiles either that they couldn't already do with their own ballistic missiles. All it does is to help them repel invaders.
Handing over US weapons directly to China would be a pretty big escalation, even all else considered. Seems unlikely but suppose we entertain it, I'm sure PLA would like to get their hands on an intact M1 tank or Patriot missile.
If Iran want to trade a F-14 for something you reckon PLA would do it?
I can already see in Top Gun 3 various flavors of J-31 being the enemy.
When most former USSR states left the Union, they also handed over their USSR equipment to the west. Saudi choosing to leave the Empire will be a similar moment. If US intends to make the weapons useless by discontinuing support, China would gladly take them off Saudi hands just for opfor simulation purposes. Otherwise, Saudi can operate them side by side with new equipment like Pakistan does.
If China really pushes for it they can likely do whatever they like with the Iran F-14s. But its not that relevant to the security situation since they're old, retired planes. China would mostly want currently in service weapons that can be taken apart to check for weaknesses and used during force on force training to enhance realism.