>disallow foreign military aid

Current hardware delivery aren't coming through during active hostilities. Unlike RU who doesn't have capabilities to deny vast UKR + EU land logistics, PRC can proactively deny air+maritime around TW. Optics of PRC allowing foreign powers to actively support seperatist gov on Chinese territory during active Chinese civil war which includes TW soil and 12nm territorial waters is not something tolerable for domestic PRC politics. Or that TW cannot be supplied during war time without invading legal Chinese soil something PRC sovereignty politics won't allow due to history. That's the significant political consideration, UKR supporters moving supplies through soverign UKR territory aren't legal belligerants, supplyiers of TW that necessarily involves what PRC percieves to be invading Chinese territory are.

>imply a global conflict

Yes as does blockading PRC, that's where the entire US can choke PRC strategy falls apart, it now feasibly escalates to CONUS war. If US blockades PRC (i.e. US initiates legal war), then PRC reciprocating attack on CONUS no longer falls under art5. Of course NATO could still join, but with what hardware, based where? Even G7 boycott weatherable, this isn't 2010s when PRC was relatively export dependent, export GDP to west is now barely 10%, with mutual knock on supply chains. Most of G7+ are MUCH more trade dependant than PRC as well and will hurt more, i.e. even least trade dependant US is going to have to contest with trillions lost in advanced industries because TW leading edge semi gone, and likely crippled domestic US semi because they will still be dependant on east asian semi supply chains that's not going to survive a broad war. Meanwhile most of that hardware is already sanctioned / denied to PRC. This doesn't suggest PRC wants a broader war, but broader war does open up host of strategic reach goals and opportunities that asymmetrically devastates PRC containers.

>before they had indigenous weapons and while they had a nuclear protector

Yes, PRC had no issue tussling with nuclear powers while not being nuclear power herself. And she tussled with her alleged nuclear protector USSR as well. Multiple times + other geopolitical drama, all less important than TW.

> civilian targets are targeted

This excludes TW civilian targets (and likely strategic targets on JP, SKR, PH, AU etc), as old addage goes, is US willing to trade nukes for Los Angeles over Taipei. Proxies can fight to the last man / brick. If US doesn't want to escalate to nuclear with PRC then the point of CONUS retaliatory strikes is to deter US from dirupting PRC civilian homefront, which includes PRC shipping. But really I'd say esclation ladder jumps near nuclear once homeland attacks, even if just military are happening, which many US war planners still think is allowable - plenty of entrenched hubris thinking unilaterally attacking PRC mainland targets would not invite proportional retaliation on CONUS or other belligerants against PRC. Chance of US blockading PRC or hitting mainland PRC increasingly unlikely as PRC fills out conventional CONUS escalation rungs. Intervention calculus changes when there's homeland vunerability, this includes US allies whose critical infra is vunerable to PRC global strike as well. If anything home land vunerability limit nuclear escalation by stopping escalation at critical conventional rungs and limit broadness of response. Alternatively this is also a world where conventional escalation can reach near existential levels of mutually assured destruction. Imagine escalation ladder with rung where US+co refinery goes boom and first world economies simply ceases to be viable. That's what's skewing escalation dominance towards PRC over core PRC concerns over TW, which starts at preventing foreign aid to speed up TW collapse/capitulation.