Fasauceome

Yeah I am thinking you might want to reach out to Corsair support and see about a replacement cable for this power supply. Their customer care is pretty good and they might replace your cables free of charge.

Are you certain that this power cable is original to this power supply? That 8 pin should absolutely go into the PSU.

I've been using a program called OpenRGB recently https://openrgb.org/ But gigabyte has Aorus RGB software that you could use to control the lights as well. Both programs are free so it's your choice. Some gigabyte boards have RGB control in the BIOS but I think it's AORUS motherboards only and the DS3H is one of the lower end gigabyte boards in terms of features.

building a keyboard is pretty niche so you probably won't be encouraged to do so unless you really want to. What is your budget and what currency?

yes, you will want to strip down the system outside the case to just CPU, ram, and motherboard with power supply to do the most troubleshooting you can.

HDD LED cables are not required for any function of the PC aside from the LED itself

on eBay compare prices on Ryzen 5 CPUs that you can get your hands on. Even first gen Ryzen like a Ryzen 5 1600 or even a 1400 is a big jump in performance. Used ram is a good idea, but new motherboards are not a bad idea. You can still get a new B450 motherboard for a good price https://it.pcpartpicker.com/product/Nh2bt6/msi-b450m-a-pro-max-micro-atx-am4-motherboard-b450m-a-pro-max

changing the CPU will require a motherboard change too, since you are still on socket FM2. Are you considering the used market? There are some pretty cheap Ryzen options that would be a pretty massive improvement.

do you have a friend or sibling's PC that could lend a power supply to test with?

there is a chance that the power surged while you were away. Is this PC plugged into a surge protector?

I will assume you already did the lowest settings to see if that makes a difference When you updated the graphics drivers, what did you use? The easiest way is GeForce experience, but we might also want to start from scratch. https://www.guru3d.com/download/display-driver-uninstaller-download/ Download this program, and run it to remove your nvidia drivers. Then you can download the latest drivers once again to see if that helps

Elden ring would be an unplayable mess if the intel 630 was the rendering device. If you are actually getting decent performance, I'd more likely believe that the 0% reading was in error.

These are both pretty good choices for your intended 1440p 144Hz performance target

If the screw that is in there right now has the proper threading for that screw hole, looks like you are set up properly for this. Having a hard time finding an unboxing video that showcases the exact screws that come with this board, but I doubt they would have two different types of screws that fit in the same hole so I think you have the exact one you need.

What specific motherboard did you get? All motherboards with an M.2 slot ought to include an M.2 screw