The Pook

Seasonic Prime Titanium are fantastic units, but I don't really know much about the Dark Power Pro.

Because it's not? i5 is the minimum requirement, not the recommended specs. Regardless, recommended/minimum spec requirements are useless if they don't specify what resolution/settings they're aiming at. https://www.battlefield.com/games/battlefield-1/tech

If you're patient you can often grab Philips Fidelio X2s / X2HRs on eBay under $200 or they're $220 new on AliExpress. They're only 35ohm so you can honestly get by without a DAC.

Easiest way to test a PSU is to swap it out for a known-good one. There really aren't any consumer tools that can that aren't insanely expensive (and before someone links me to tools like this, they're not going to diagnose issues past rails being out of spec. rails can be within spec while the PSU is still faulty).

mud tower, lol Fractal Design Meshify C.

Noooooooo, do not get the P400/P400S! The front panel means almost 0 intake air. I had a P400S and just bought a Meshify S2 to replace it because temps were so bad. The original non-S2 Meshify is fairly cheap and is great, +1 for that.

Well you won't see 8.1GB with 8GB, it's just going to put the excess >8GB on the page file. Your CPU is at 100% in Battlefield is because it's a quad core. My i7 6700 was nearly always pegged at 100% too, even my 9900K is 60%+. BF is fairly well multithreaded.

Buying the exact model is ideal but you don't have to. I'm running two different 2x4 kits and they run in dual channel fine at 4x4. Monitor your RAM and page file usage. Can do both with HWInfo. If during the spikes you're >8000MB RAM usage and your page file usage is abnormally high, then yeah, that's your issue.

AsRock Z390 Taichi/Taichi Ultimate (same boards except Ultimate has better WiFi and networking) or Gigabyte Z390 Aorus Pro get my vote and they're reasonably priced. You don't need an 8+4 for an OCed 9900K unless you're on sub-ambient ... 8 pin is enough.

Good pair of headphones + ModMic/MiniMic/V-Moda BoomPro > any headset you can name.

Self control is free.

Grab an X-Ray pad: https://shop.x-raypad.com/product-category/products/custom-mousepads/custom-extra-large-mouse-pads/ I got one before Christmas during a promo and paid like $25 shipped to my door for a 26" x 14" with custom artwork on it and it's great. Even their normal prices are very reasonable.

Monoprice Modern Retro. They're a bit cheaper if you buy directly from Monoprice but they're on Amazon if you have to buy from them. A pad upgrade is a good idea since they're a bit tight and it can be uncomfortable but up to you.

If you're willing to buy from AliExpress and wait for shipping - SHP9500s popped up after being OOS for months and they're great headphones. I bought mine for $55, but I gladly would have paid triple for them. If you want closed - ATH-M40Xes... or if you want to go a route less traveled, find a pair of Monoprice Modern Retros and throw on a pair of Brainwav XL pads. Under a $60 investment and they have comical amounts of bass without the highs and mids being wonky.

your GPU won't boost to the same clocks at 70c that it would at 40c and it won't boost if it determines the task that it's performing isn't worth running at higher clocks. My 1060 gets beat by my old 950 in 3DMark06 because of it. you can lock it to a higher P state with MSI Afterburner but you're likely to use more power and put out more heat for little to no real benefit. you can do that by pressing CTRL + F in the bottom graph portion of Afterburner, click the highest point on the graph that opens, close the window, and click apply on Afterburner.