Check the games you plan to play to see if they have good/any SLI support. DX12 moved SLI support from the drivers to the game developers, to a large extent, and SLI is less frequently well supported or beneficial at all. Main reason to go multi card these days is for GPU compute work.
What's your backup routine? RAID 1 improves uptime but is not a replacement for backups as corrupted or infected data written to one drive will be written to both, and a board level failure could still kill both drives at once (i.e. power surge). I rotate 2 USB HDDs for weekly backups and use OneDrive as my offsite copy for pictures and documents. If it were me, and I could replace a failed OS drive out of pocket within a day or two while waiting on warranty replacement, I'd go for a single larger NVME rather than a RAID 1 OS drive. 2TB 970 EVO looks to be about 30 more than your current 3 drive setup but gives you a single drive to manage or partition as needed and all of your space is NVME fast.
https://de.pcpartpicker.com/product/vMMwrH
I recommend either going with a larger closed loop cooler or sticking with air cooling. The water cooling units mainly pay off in noise/temp when you get up to the 2x120mm or 2x140mm radiator units. I have a Ryzen 2700x system using a Corsair 2x120mm and an i9-7900x system using an Enermax 2x120mm system. Both of these perform great and are fairly quiet. Just make sure your radiator is sized to match the mounting locations available on your case. The Corsair 500D case you picked can take a 2x120, 2x140 or even 3x120mm radiator size. Also worth considering a Corsair closed loop unit just to have all your RGB running on the same protocol and controllable by a single software package.
Break down of cooler type advantages/disadvantages:
https://www.youtube.com/watch?v=OY2KeVwtbYU