tl;dr
I will backup data and re-install, but not because this is unfixable. Thanks @lew and @dalto for helping out - chmod on a single file or directory was the right answer here, the rest is addressing corrupted packages as it is in the other threads.
For anyone that ends up here in the future: Remember to be careful with root privileges, kids!
/tl;dr
Now, for the curious - I started fixing things one by one, but I think I have even more corrupted packages than @ArchMonkey from the other thread. That, along with my stupid mistake of chmoding my root directory to 777, made it a bit too much effort for a machine that I’ll be leaving behind very soon. Should this have been a computer I’d be using often or as a main one, I would have persisted, but sadly it is not.
It would have been a very interesting exercise to fix both the corrupted packages and the 777 issue, so I’m not too happy I’m leaving those behind unsolved, but you have to consider time and effort always. I hope this kernel panic while updating isn’t too wide-spread either. Not everyone is comfortable with chrooting and as we saw, even if you are, you can make a mess of it 
I say sadly, because this is my longest running Endeavour install on physical hardware and I’ll be sad to see it go. I think it’s around 3 years old and the only problem I’ve had with it was actually with GRUB, not the install itself. In fact, the recent move to Wayland and Plasma 6 improved the over-heating problems of the hardware with no action from myself, apart from running the update 
(HP Z book, awful, especially at their price range)
But this is a business machine and they’re making me switch off of it, so even if I fix this, I’ll gain no more than a few weeks with it at best. (My new options are Ubuntu, Windows 10 or Mac, which is my lowlight of 2024… for now - all three are at a similar level of bloat and annoy me greatly - sorry for venting)
I say all this to say thank you, it’s been great and I’ll continue to use Endeavour as my main Arch-based distro, just not on this particular machine any more. Great distro, great community from the start (I think I have one Virtualbox machine with the original XFCE flavour of Endeavour still running), great purple aesthetic and wallpapers 
.