Me: "Well I can't write or do anything useful, so I might as well aimlessly click. Ooh, what's this, a windows update I don't have. Sure."
Windows Update: "Problems installing KB5034441, error 0x80070643"
Me: "Well that's not useful."
5 Minutes Later
Me: "Are you fucking kidding me, Microsoft?"
So the reason why it's failing to install, is that it's a recovery partition update but it's too big for the partition, so you need to manually resize your partitions to fix their stupid fuckup
Microsoft: "Here's a PowerShell script that does the Hoop Jumps for you!"
Me, suspicious: "Yeah, okay, sure"
The PowerShell script: "Sorry, can't resize your root partition, not enough space for this 250 meg shrink"
Me, staring at the 8 gigs free: "How? Why?"
The PowerShell script: 🤷♀️"iunno man, just, can't, can only get 16 megs"
Me: "... is it a fragmentation thing? altho that shouldn't be a fucking problem with ssds"
The PowerShell script: 🤷♀️
Amazing.
I've finally been able to do this.
Not with the powershell script, but with the manual guide.
And not in one go, because of course not.
You see, if there's any fucking windows system file in the way, it can block the resize - without it telling you this is the reason, of course.
This includes page files, hibernation files, and a whole lot of random bullshit management things you have no control over, so your best bet is to hope defrags, turning off as many things as possible, and enough reboots... will get you where you want to go.
After way too long, it finally deigned to tell me I could shrink my primary partition by like, 3164MB, which was luckily enough to add the 250 it wants (I gave it 512, because fuck it, I'm not going to go through this again)
May this help someone else that's got the same fucking stupid update stuck there since the start of the year.