9900K is great for Adobe software. It even trashes the 9990XE in Photoshop, and those are sold in secret invite only auctions with Eyes Wide Shut dress code and cost 2400$ with no warranty.
Do you have both "cpu"-power connectors connected? 8 and 4 pin?
You could also try setting Windows to max performance mode from power-savings settings.
Fuma.
Also check out Thermalright Macho rev. b and Le Grand Macho. Noctua and Be quiet usually overpriced.
https://www.techpowerup.com/reviews/Scythe/Fuma/6.html
Just go for 2600 if you buy a gpu.
H200 is huge for an itx-case and designed water cooling in mind. You can find atx-cases that are similar size and a lot better for air-cooled gpu:s.
http://www.raijintek.com/en/products_detail.php?ProductID=53
Just plug it into the pc, turn it on and then test? That video is 3 year old and so bad that you cant even see if there is a jumper in 24pin connector.
1660ti is a lot faster and that 100$ difference comparing new 1660ti and used rx580 is just ridiculous. It should be more than that while both cards are new.
If you need fans, buy a 5-pack of Arctic p12:s from amazon for 28$.
https://www.amazon.com/ARCTIC-ACFAN00137A-Value-Pack-Pressure-Optimized/dp/B07HC782D5/ref=sr_1_1?ie=UTF8&qid=1551162998&sr=8-1&keywords=arctic+p12
I guess it depends on what you do with 9900K. Oc it to 5GHz on all cores with avx loads and you'll almost break 250W with just the cpu. Custom 1080ti can draw over 300W when oc'd with peaks over 350W.