So lets start with the cache raid its built in but a little hidden on the option:
FAQ for unRAID v6
You can choose raid 1 (default), 0 or 10 and raid 5/6 are in beta (only for brtfs cache) they added a nice gui selection in 6.8.3 recently I did it via command line previously.
I currently have 4x 500gb sata drives I just feel like its a bit of a waste of space both physical and digital, I didn't want to sacrifice a 5.25" slot for a 4x ssd enclosure when I have an extra x8 pcie slot (and the board supports bifurcation) so the NVME adapter board was the way to go. These 860 evos were left over from when I migrated from my 1635AX nas so didnt pay any extra.
Yes, I highly recommend the containers, what I did was first build some VMs running containers to get familiar with how containers work and to migrate Plex, unifi controller, unifi video, etc and keep all of my current watch counts/lists etc the guides are pretty straight forward (but it takes a while to copy Plex due to tons of tiny metadata files). From there I copied the container files and configuration options into a new template on unraid and they started right up pretty much with only a few minor tweaks.
On the CPU thats really up to you, frankly its not bad each box is using about 150w 95% idle, not horrible, but definitely feels like wasted usage when I dont need that much power that often and a much lower usage CPU is cheaper and still decent compute. I originally was thinking I wanted the highest frequency possible with good cores for the VMs, then containers happened.
I actually converted my array back to raid 1 from raid 10, didnt notice a bit of performance difference on my primary box with those sata drives.
I just did the NVME because e-peen and they drives I got have better endurance than sata, if the sata is working and doesnt feel slow keep it IMO.
My primary and secondary are clones from a array perspective, the cache is similar (both raid 10), I'm running DirSyncPro in a container to clone my media changes nightly. It was one of those when I transitioned to unraid I needed the array built up first, so I had to buy all the drives anyway, when I retired my qnap nas I then had 10 drives with no use, so build another box!
(sitting at about 55TB used so not many externals available for that, gonna need to add another pair of drives soon too).