Hello! Busy weekend of work & family commitments, but I've finally got time to sit down and reply - thank you so much to everyone who replied to my plea. I've been playing TS2 since launch and have never had such a bad scare in terms of breaking things (including the time I used the first iteration of AGS and had to manually registry edit every time I used it as my old PC would freak out).
So, the good news is - not only have I fixed things, but all I lost was placement of a few embankments, which is easily fixed. I'm 99% sure there was a conflict between Criquette's Neighbourhood Lighting Remedy and Gunmod's Radiance Lighting System, both of which I've now removed from my setup.
To fix/diagnose things, I pulled everything from my download folder, then ran the Origin 'Repair Game' feature. This broke the graphic fixes as in the original post, and despite restarting my laptop, nothing stuck - so in classic 'definition of madness is doing the same thing and expecting a different result' mode, I repeated both steps - and amazingly, the third time, it worked. Maybe Origin just got annoyed of me resetting things? Idk. Each time I reopened the game, I used Belladonna Cove as my 'testing' 'hood so I didn't lose any deco I'd placed, but I forgot this the last time I reloaded to check - more on that later.
I then carried out the time-consuming process of checking I didn't have any other errant downloads causing issues, so I added a folder of CC at a time back to the download folder, opened the game to check for any flashing, repeat. I left my mods folder and Criquette folder until last (dunno how everyone else sorts their CC, but I'm a firm 'folder for each creator, then subfolder' person. Yes, I know it slows things a little but my OCD likes it.) For these two, I removed any shader mods as they seemed top cause the biggest conflicts, then reintroduced things. As I've removed Radiance Lighting, I've also left out the 'hood lighting fix I suspect caused the issues (as I'd downloaded the version meant to work with Radiance - going to try the vanilla EA version tomorrow, so fingers crossed).
Whilst this was a long process and has left me needing to reinstall Scriptorium & associated CC like custom stairs, I have a working game again and only lost the previously mentioned embankments when I loaded a custom 'hood without adding them back into the Download folder. Hopefully this will save another person having similar issues!
Thank you to @criquette-was-here, @captainnikki, @cozy-sims, @origami-penguin , @lilythiums, @hankofthesims, and @acornsandthimbles-simblr for replying and offering help, I greatly appreciate it. One of the biggest reasons I've never quite been able to leave the TS2 community is how helpful and welcoming everyone in the community is, and 17 years hasn't dimmed it at all!