OP:
If you don't feel like opening it, just get an external USB3 SSD, and plug that in and set it up to be the new boot drive.
You'll see read speeds in the 420-430MBps range.
Although this is nowhere near as fast as the newest iMacs with built-in SSDs, it will still make it feel "snappier" and much more responsive.
You could buy one that's "ready-to-go", or buy a "bare" 2.5" SSD and an external enclosure like this:
SABRENT 2.5 Inch SATA to USB 3.0 Tool Free External Hard Drive Enclosure [Optimized for SSD, Support UASP SATA III] Black (EC-UASP)
tinyurl.com
(just snaps together)
I reckon 500gb would be a "good enough" size to keep you going for a while.
You could still use the internal for storing large libraries of movies, music, etc.
You could use CarbonCopyCloner (which is FREE to download and use for 30 days) to "clone over" the internal drive to the external.
If a 500gb SSD is "too small for everything", use CCC's option to clone only "some" items to create a "selective clone". You do this by "un-checking" things that you wish to leave behind.
One advantage of booting from the external is that in the future, when you get a new Mac, it takes 10 seconds to disconnect the SSD from the old one. Then use it for your "migration source" to the new one, and afterwards you can "re-purpose" it for other uses if you wish.