An update on this, I wrote up about 3,000 words of documentation and took it to a repair place in town. Get a call the next day, it's working fine, he leaves it running an intense Fuzzy Donut test overnight. Call back in the morning, it's fine. No sign of the PSU being banjaxed either. I'm relieved. The GPU isn't faulty in some fashion. Sounds like all it needed was DDU, a CMOS reset and a bios update. I pick it up, bring it home, plug in power and displayport cables and...the same. Nothing happens.
I try a different power plug. Same. I start getting very upset. I try something as a hail mary and, would you believe it, nothing was wrong with any of the parts in the first place at all. Do you know what the damn problem was? It just won't boot if it's only got DisplayPort plugged in with no HDMI. Plug HDMI in to the card, it's totally fine. I start laughing at how absurd it all is. How was I supposed to expect that, my old 1660 was fine with just displayport.
That's something I'm going to be working on fixing (from what I gather it's down to the DP cables themselves, but I didn't experience this on the 1660) but it's absolutely workable; it just does BIOS over HDMI then switches over to displayport once I get to Windows, TV auto-switches. Some of my other issues still remain (seemingly RAM-related), but the GPU is fine. That's my wait mostly over with. I'm still setting up my new OS but I can't wait to give it a test.
As an aside, has anyone else with 3080/Ti models (specifically the strix 3080Ti OC in my case) noticed anything with coil whine? It would make quite a lot of noise when running Heaven Benchmark, but it's whisper-quiet otherwise and Heaven has it running 100% achieving around 300fps. There's also a chance the whine might quieten down with time based on some anecdotes, and I'm usually wearing headphones. Have yet to test it with games running at less intense framerates.
Don't know if other said this, but it might not be that it's not booting, but only that the screen is blank while it's booting up, I've had this happen depending on what cable type I am using or what GPU I have in my machine at the time.