"As I recently bought a 27" Late 2012 iMac with 1TB HDD, I have a similar dilemma in regards of the HDD/SSD. What is the consensus(if any) on the TRIM support on the externally connected(via USB) SSD's?"
I booted and ran my 2012 Mini from a USB3 SSD for SIX YEARS -- right from the first day I took it out of the box. It's still running that way on the back table.
TRIM was NEVER an issue.
To extend the life of a 2012 iMac for a few years, a USB3 SSD set up as an external boot drive will be "all you need"...