Iwo Jima's Central Field has a 2650m runway
Which means any PLAAF aircraft can take off and land from there.
I'm sure they can have Y-20s fly over and unload HQ-16 unit as well as SHORAD + all the gears and construction materials and such. Bring in the material to set up a temporary pier. You can have Y-8/9 special missions aircraft land there and increase their reach. As well as UCAVs. You can have YY-20 fly out of there to refuel aircraft looking to attack Guam or even Wake Island.
You can bring in construction crew and material really fast to build up protective hangars. Expand fuel depot and setup communication center and radar stations. Anything can be done if PLA plans it out well and do things efficiently. Remember, there were over 20000 IJA stationed there during WWII and America sent in > 100k troops. It's not that small of an island. There will also be air defense offered by fleet nearby.
On the topic of Iwo Jima - Assuming that China possess the capability of both capturing and holding the island in the future, technically speaking, China could erect an early-warning radar system that can expand China's missile warning horizon much further eastwards. A three-faced LPAR radar on the island is the best option.
However, given the relative isolation of the island, alongside its wide exposure to sea-based (Virginia SSNs and SSGNs with Tomahawk and/or CPS) and aerial-based attacks (standoff missile attacks by bombers and/or carrier-based fighters) without nearby landmass support to provide in-time cover, especially when the war is still very much ongoing - LPAR-type system is out of the question.
That means until the war ends with China still holding onto Iwo Jima - Only less-expensive facilities and systems could be built there. Something that can both be erected quickly, eating comparably fewer resources, and can be repaired, replaced or rebuilt quickly in case of destruction from enemy action.
Of course, this also brings the question on whether it's better for the PLAN to procure something of a sea-based early warning radar platform that can move around.
One possible option would be something akin to the SBX-1, which is an X-band AESA radar based on a modified oil rig platform that can be move around (although with a very slow top speed of less than 10 knots). If Iwo Jima (or any opposing island) can been captured, then these platforms can be moved to be stationed there. The massive size associated with such platforms mean that they are more stable and capable of housing larger systems which would be deemed impractical for warships.
Another option would be a missile tracking and defense (BMD) ship, i.e. something similar to the ASEV pair which are currently being built for the JMSDF. They are certainly smaller and less capable compared to the aforementioned oil rig-based and land-based systems, but their speed and mobility means that they can be very flexible in terms of deployment, alongside being superior in terms of survivability.
I'd say that the 1st option would do good for basing at Iwo Jima (or any captured outlying islands in the 1IC and 2IC), while keeping the 2nd option for flexible, wide-reaching early-warning coverage.