Prakticaljoke

It'll work, it's just that it'll run at 3.0 speeds.
Rocket Lake (11th gen) chips will enable PCIe 4.0 support on Z490.

Yes, its backwards compatible, so it should work. 

It will work just fine m.2 is forwards and backwards compatible. Yours will just run slower not that it matters for your use case anyways

Where do you draw the line, though? You'll not run into any issues whatsoever with 16GB of ram for gaming (usually). If you play some 'out-of-the-ordinary' games that have high memory requirements regularly - then I could also understand the 32GB. 64GB however is just insane. If you don't mind the price, why don't you max out the MoBo and go for 128GB? See where I'm going with this? Same goes for storage and PSU.

With the mindset of always having the best components I'd much rather buy reasonable components that actually matter and use the money that you saved to upgrade to RTX 3000 or Ryzen 4000 in the future.

In my experience, a FireCuda is not worth it. You don't need 500GB for windows, you need 100GB, max. Use the rest of the SSD for something better. You choose the right M.2, but unless you are saving stuff like adobe creative cloud to the FireCuda because you ran out of space on an SSD, you won't see a difference. Never use an SSHD for generic, under used secondary or tertiary storage, bc it is a waste of money. 
 
No the 9900K won't fit LGA 2066, that is an X-series socket, and you are wasting money on gaming. X-series won't see a bit of difference in gaming. There are plenty of high end LGA 1151 mobos to choose from. Just look up Z390 on sites like PCPP or newegg/amazon. 

Go and return ur CPU and mobo, they are not what you need. Go with i9-990K or a Ryzen 9 3900X and a compatible mobo (look through reviews and find the one with the features u want). You don't need 64GB or RAM, upgrade it when the world has gotten to a point when 32GB of RAM just isn't enough. You may want to go 10th gen intel, but don't go X-series, just go with a K-series. 

Interesting choices for displays.
 
Don't bother quoting a US price. US pricing has no relevance to you.
 
As for the parts of the build:
PSU is monstrously overkill, and a complete waste. You should have gone with faster memory. If you're only gaming, the CPU is a waste. 64GB of RAM, again, is unnecessary. 32GB would make sense depending on how you use your PC. All in all, there's a ton of wasted money here, on parts that only make sense if you're trying to have the biggest epeen, which IMO is kind of sad.
 
The 9900k makes absolutely no sense now that the 109700k is almost here.