Would I clone the HD and put that on the SSD?Silicon Power make a 240GB Thunderbolt SSD. Just needs hooking up to the Thunderbolt port, and I used a Twelve South Rack which mounts on the iMac leg to hold it. In System Preferences select as the Boot Drive.
Does TRIM compliance matter?I use a USB 3 enclosure for a 1TB SSD external boot drive on my 2015 5K iMac. For most SSDs, the only advantage of the Thunderbolt enclosure is TRIM compliance.
Quite Interesting this point. An 3.0 USB Case with a SSD will run faster than my fusion drive?Don't overthink this.
I use a 240gb external booter for my late-2012 Mac Mini. I have the SSD partitioned. One partition is my "boot partition". The second serves as my "Main" partition, where all my data files (going back 30+ years!) are stored.
I back both of these up to similar partitions on the Mini's internal 1tb drive.