>already factored

I think not yet, or at least not seriously. The notion that US rivals (likely just PRC) can execute major conventional attacks on US infra is not currently accepted thinking. Only newest strategic/policy writing from Congressional Research Service and handful defense think tanks have a few lines acknowledging PRC is pursuing PGS capabilities which was only formally put on paper in PLA's 2020 Science of Military Strategy. And since it's not demonstrated capability outside of what's hinted by PRC's hypersonic glide FOBS test in late 2021, folks haven't spent too much brain cells analyzing/gaming out the potential. The renewed fixation over PGS is just getting started. Right now there's nascent acknowledgement that such capabilties are something PRC is pouring a lot of resources into pursuing. I think we'll see more serious discussion over the next few years, especially if PRC does a few more visible tests demonstrating relevant capability or rocket force adopts new hardware which at PLA speed, will shift strategic reality rapidly. Of course it could all be vaporware. In the meantime:

>US won't first-strike mainland China

PRC hawks and some warplanning/gaming currently are entertain mainland strikes. Consider that sinking PLAN invasion fleet while they mass (within PRC territorial waters) is tantamount to hitting targets within sovereign PRC territory, or that PRC considers TW literal Chinese soil. It gets messy. In general I agree hitting homeland is very far high up the escalation ladder. But until PRC has proven ability to retaliate on CONUS with proportional response, US planners still have option/incentive to plan with mainland strikes in mind, reasonably believing PRC won't escalate to nuclear due to NFU, at least not until PRC finishes current nuclear build out. We're in this latent space where it's easy for hawks to argue PRC doesn't have comprehensive conventional or nuclear deterrence against US conventional strikes on PRC mainland with acceptable costs (US homeland unscathed). Meanwhile the signs are pointing to PRC being able to close both vunerabilities/gaps sooner than later. All of which is to say, currently, US might strike mainland, or at least PLA has yet to comprehensively deterred US from doing so. Credit to RU having 6000+ nukes vs PRC racing to 1000 because ~300 (if accurate) simply wasn't sufficient.

>huge stretch ... wouldn't already have gone nuclear

Yeah, IMO that's the point, viable massive conventional strikes against CONUS introduces new layer of MAD that will now ripple out to conventional actions, i.e. if US blockades PRC shipping, without ability to rapid strike CONUS, PRC relegated to run blockade and escalate to very unfavorable IndoPac naval shootout, and if situation existential, could lead to huge escalation to tactical nukes on carrier groups over international waters if things get desperate, then hard to say how things spiral. With prompt global strike, PRC will have leverage to run blockade and threaten to hit fast supply ships replenishing in Yokosuka (there are currently only 10 sustaining entire USN) without which carrier escorts and carrier group become single deployment assets with days of useful endurance. Or any carrier undergoing maintenance in drydock. Or other critical US assets, from Pinegap in AU to drilling platforms in gulf of Mexico. Lots of steps before CONUS. The purpose is to create escalation steps to bridge gap before massively disruptive homeland strikes or going nuclear.

>which case the incentive becomes to strike first and strike as hard as possible

That's already the current incentive of modern war, incredible advantage that bias towards shooting first since exquisite major assets (highend ships, subs, bombers) are difficult (if not impossible) to replace in a timely manner in ongoing war, hence anxeity over PRC preemptive attacks on regional US forces in TW scenario. IMO both sides developing PGS hypersonics is establishing credible conventional second strike that will disincentivize large scale peer war. The implication being, this massive navy we both have is really only good for bullying smaller targets because it doesn't matter who sinks whose navy first, the survivor's fleet will be scrap the second the pull into port since conventional missiles in hardened bunkers are credible second strike options that ensures nobody ends up with a blue water navy to maintain their security interests at all.

The way I see it: right now US is the big kid on the block with a fancy gun, dictating terms to everyone in the neighbourhood. Her closest big kid rivals have knives and can't get close. The big kids also have suicide vests but no one wants to use them unless on death's door. PRC plans to start a knife fight with TW who is armed with a tooth pick. US plans to step in with her fancy gun but at a distance where she's least vunerable. PGS hypersonics gives PRC a shitty gun, but it's still a gun and provides significant more deterrence and capability than the best knife. Now neither gunslingers want to fight each other, US has to reconsider whether stepping in a PRC vs TW fight is still worthwhile. A fight between gunslingers can still happen, but it's less likely than a knife vs gun fight. Eventually, both side settles with giving each other space while bullying those armed with knives and toothpicks, trying to prevent them from also acquiring guns and suicide vests. US will get a better gun, maybe start wear increasingly better bullet proof vest (ABM) while her rivals work on AP bullets, but she's not going to be as secure or have as much coercive power as when she was the only kid with the gun.