Sardz

Easily get away with RAID 1 or RAID 10 as those forms of volume fault tolerance don't require parity calculations that would otherwise be bottlenecked by the CPU depending how much data (throughput) is being written to the NAS as an NVR. You can try RAID5 (requires a minimum of 3 drives in the array) and test before committing with full production of the unit.

For a simple NAS an old dual core CPU is fine. I've been running a Core 2 Duo machine as a NAS for many years now, and it has zero issues with a gigabit network. 

microcenter had 9700k for $199 couple weeks ago, but I doubt that upgrade would have significant benefit on the games you just mentioned, I could be wrong though you should look up benchmark on youtube too.
These games tend to be GPU bound and upgrading to a more powerful GPU would be more reasonable.
CPU upgrades are usually unnecessary, unless you are league/valorant/csgo players.
If you do plan to play competitive titles in the future, ryzen 5000 is the way to go otherwise I do not recommend upgrading your cpu at the moment.

They only support 8th and 9th gen Intel Core processors. Any 400 series board from Intel will support 10th gen, such as the 10400F, and 11th gen Rocket Lake coming out towards the end of Q1 2021. 

Depends on your definition of long term. If you plan to keep the system for 5 years, you should be fine, given that you have a decent cooling setup. Long term, those components will age faster and therefore degrade more than under rated conditions. If the system does anything mission critical, any kind of OC is generally bad. If the system is supposed to run 10 years of more, avoid OC on your GPU can only OC the CPU as far as you can without touching voltages. An all core boost that most motherboards will offer may be the extent of your OC adventures if you plan on running that PC for as long as that. 
Obviously a non overclocked, adequately cooled system may run for more than a decade but at that point you won't be playing newer games anymore, unless cloud streaming becomes a thing and everyone has 1 Gbit internet in their homes.

Depends. If for example you were giving the cpu insane amount of voltage then it will degrade over time. As for the GPU, it's firmware limits it to being pushed to crazy levels, so for the most part if the overclock is stable it shouldn't die. 

all 2666mhz is out of stock, so ill just buy the 2800mhz Gskill.

all 2666mhz is out of stock, so ill just buy the 2800mhz Gskill.

it'll just run at 2666mhz, it should still work.

Yup, it does. I have the same unit. It does however lack a second EPS 8-pin cable for the CPU, not that it's needed unless you get into the intense overclocking territory.

It has 4 6+2 PCIe pin connectors. All the cables most likely comes in the box. 
You can run your 2 1080ti of this thing

No.
 
If you wish to migrate, then you don't re-install. The migration brings verything over to the SSD. 
Pro: All your stuff is there
Con: So is all the cruft
 
If you do a fresh install, just do it on the SSD and don't worry about what's on the HDD

Could you? Probably, but you'd be dragging all of the settings, data, and everything else from your laptop to a new PC. I would highly recommend starting fresh with a clean install instead.