Boomy Beatle

Agreed. I forget which one, but an Austin Evans video showed a PC that didn't boot because a specific pin on the HDMI connector caused a weird power spike. Changing the cable fixed it, so it might work for you as well.

Use DDU--Display Driver Uninstaller--and uninstall the old Radeon drivers. It might be causing a conflict. My only other guess is the PSU. The 980 is more demanding than the 470, so upgrading may have caused you to more easily trigger shutdowns because of the increased power draw.

People like him are just throwing everything at the wall and seeing what sticks. He may have known you probably wouldn't fall for it, but there's no reason he wouldn't at least try.

Take it off completely and check the cold plate (the metal that makes contact with the CPU). There's a chance you may have left the plastic peel on there.

I would go for the 1000W, at least. 850W may be good enough to handle the 11900K and 3090 on their own, even during those quick power spikes, but with all the extra fans and lighting you plan on adding, you'll definitely need some room to grow. At that point you're more likely to find higher efficiency PSUs anyway. Gold is good, but it seems like it's quickly becoming the default.

Maybe try eBay. You might not get something official, but for a piece of metal like that, it won't really matter.

When you say you checked the mounting, did you take the cooler off the CPU?

Remove the CMOS battery, leave it out for a few seconds, then pop it back in. Remember how it was oriented though, otherwise your BIOS won't save anything until you put it back in properly. This probably goes without saying, but do this while the PC is off.

For an air-cooled Alienware? Yeah, that's good, especially for peak.

Sounds like you did turn on the PSU switch on the back, so we can cross out that little mistake first. Your next course should be to reseat the RAM and the cables. Take off the side panel and keep it off, then switch off the PSU switch, but keep the PC plugged in. Go through the RAM sticks one at a time, then the power cables, trying to turn on the PC each time. Do it slow like this that way you know for sure what fixed it.

Score of a lifetime right there.

No, it'll work fine. The BIOS takes care of the most basic operations and system monitoring of your PC. The most common cause for anyone to really "need" to update their firmware is when they're putting a newer CPU into an older motherboard that supports it, but hasn't yet been updated. Other times some features will come out that you may want if you're more of an enthusiast. Should you ever decide to update the BIOS in the future, make sure you're not in a situation where the power can go out. You will brick your system.

I was just trying to match core-for-core, but yeah, those are some great points too.

Given your use case and desired upgrades, some kind of thin-and-light would be perfect. The Dell XPS line is highly praised and should easily fit into your budget if you spec it out well enough. They can come with lower-end GPUs, they're lightweight yet well-built, and they're pretty much designed to be docked. I'd recommend getting 16GB of RAM, although that's a personal preference of mine. 8GB has essentially become "bare minimum" in my eyes. Don't go over though, because at that point, you're wasting money considering your workload.

One possible solution is to simply reseat the GPU in the PCIe slot and doublecheck the power cables.