The luck didn’t help haha, I never got that light again, except for occasional split second flashes. I edited some info into the original post, but long story short, after lots of fluffing around in the end it was the motherboard. I got a new one and now I’m back up and running!
For the record, it was definitely on. I ended up buying a new PSU to check, and it didn’t help. I then bought a new motherboard and that fixed it. So case solved!