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.
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.