..however I do think you are slightly overestimating the utility of those reclaimed islands at this stage... and potentially gravely underestimating their vulnerability to cruise missile strikes and air strikes.
That's a matter of opinion.
If left to fend for themselves, those islands would be massively vulnerable.
However, with the PLAN fleet backing them up, it would be exceptionally difficult and costly to inflict enough damage with missiles and air strikes to knock them out of the fight and keep them out.
With a PLAN fleet nearby, those islands will enjoy the kind of multi layered air defences the equal of any CSG. And that's just from the PLAN fleet elements.
In addition, they could put dozens of LD2000 CIWS, AAA, short and medium ranged missiles on each of the islands and just exhaust and attacker's missile stocks.
Any missiles that do come through will cause damage, but unlike a ship, that damage could be easily and quickly repair, with replacement units brought in for anything equipment that gets taken out.
The islands would form unsinkable missile spungs, able to defeat and absorb an order of magnitude more missile and bomb strikes than any enemy fleet's entire munitions inventories.
In the history of modern naval warfare, it is hard to find examples where island strong points have been neutralised by anything other than direct invasion or blockade.
Therein lies real vulnerability of those islands, as are the same as any island campaign in history - the supply lines.
If an enemy could cut the supply lines to those islands, those islands become irrelevant even if the garrison keeps possession of it.
Those islands and the PLAN fleet, as well as PLAAF assets that will be forward deployed, not to mention China's ADA2 assets will be able to every effectively curb the air and surface activities of hostile forces, or at least make any air and surface attack cost prohibitive and largely ineffective.
The key to how well those islands can fulfil the role the PLA high command wants and needs them to would be determined by how effectively the PLAN could combat subsurface hostile threats behind the islands.
This is why the PLAN has ordered 056s in such numbers - that is the area where they know they are vulnerable, which is what the 056, and especially its ASW variant, is designed to address.
Those will act as your traditional sub chasers and convoy escorts, with air support from the smaller Chinese helicopter bases, as well as maybe future PLAN LHDs, which will function as fleet carriers or ASW helicopter carriers.
In terms of missiles and bombs, I am confident those islands can take anything any hostile fleet could throw at them. It's keeping those islands reinforced and supplied where the PLAN will probably have the biggest worries and take the bigger losses.
This isn't to say the reclaimed islands will have no military utility during peacetime or wartime, but I definitely would not describe them as allowing "the PLA to fight as if they were fighting along China's mainland coast".
Funny you cut the quantifying remark out in that quote. Trying to make it sound less reasonable to strengthen your counter?
In terms of defensive and offensive power, any enemy fleet assaulting those islands will face pretty much the same threats as they would if they were assaulting the Chinese mainland itself. They will have to contend with the PLAN fleet, the PLAAF top cover and AWACS support, coastal FACS as well as probably land based air defences and offensive missile batteries.
If anything, assaulting those islands may actually be harder than attacking the Chinese mainland.
If the Chinese fleet were defending the mainland, they need to place themselves in front of the coast, both because of simply geography, but also because there are high value assets they need to protect on the mainland, which are pretty much entirely absent with the islands.
That puts the more vulnerable ships up front, so every enemy missile that gets through is potentially a capital ship sunk, out of the fight, or at a minimum having significantly degraded combat capabilities.
An attacking enemy could grind the PLAN fleet to nothing with enough missiles and bombs, which will ultimately open up the coast and inland areas to direct attack, where the real damage can be done.
OTOH, with those islands, China can pick and choose what it deploys and only place assets it is prepared to loose on them in times of war.
That means they know they can afford to let a certain percentage of missiles through to hit the islands, and the damage will be entirely manageable. That has profound implications for combat doctorine, tactics and munitions expenditure, which will allow them to be able withstand attacks significantly larger than that which would overwhelm a fleet's defences.
The PLAN fleet can shelter behind the islands, so that any enemy attack will have to get past the island defences before they can even think about trying to get past the PLAN's fleets' own multi layered air defences. Which would be more effective than normal, as they will have advanced warning of the incoming attack from the islands.
The USN fleet could shoot its entire cruise missile inventory at those islands, and the Chinese will just come out of their bunkers, bulldoze the craters and unpack fresh air defences and AShM batteries from air drops and supply ships and be back in business long before the USN fleet is even back at a friendly base to restock on munitions.
The only way to make such an bombardment 'stick' is to immediately follow it up with a full amphibious invasion to take those islands.