Ok, another update.
I decided to go for the Synology 413j NAS and have directly attached it to the Gb LAN connection on the iMac. All working perfectly!
So;
Working files are on the fusion drive, transferred to the Icy Drive once finished with
All data (images, raws, documents etc) are backed up to the Icy Drive via USB3
All data (images, raws, documents etc) is also backed up to the Synology NAS
All final jpgs are uploaded to Zenfolio
The plan is to buy another Icy Drive to store offsite.
Thanks for everyones input - much appreciated.
I just need to work out a backup strategy - as in what app to use. On my windows machine I used Synctoy which was great.