Yes, cache was a singular pool, now you can have multiple pools, so the name changed.
Assuming your signature is accurate, and you are only sending and receiving data over a 1GB LAN, then SSD write speed won't make very much if any difference in your day to day usage. If you decide to enable docker containers to migrate your plex and 'arrs to Unraid, or the vm service, then you would benefit.
If you do decide to start using containers, it's easier to set them up correctly with the SSD installed when you enable the docker service. If the docker service is enabled prior to there being a pool defined for it, the system will get created on the array drives and it's a several step procedure to get them moved properly.