It's not much more than child's play to add an external SSD to an iMac, and set it up to become "the new boot drive".
ANYBODY can do this.
The iMac will run much faster once you do it.
What you need:
1. An external USB3 SSD. You can either buy one that's "all put together and ready to use", or, get a "bare" 2.5" SSD (I like Crucial or Sandisk), 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
You didn't tell us how large your internal drive is.
If it's 1tb, I suggest a 1tb SSD, will make things easier.
2. Next, connect the external SSD to the iMac.
3. Open disk utility. Go to the view menu and choose "show all devices".
4. You should see the SSD "on the left". Click on it and click the erase button.
5. You need to tell disk utility HOW to erase/format it. Choose "APFS with GUID partition format".
6. When done, quit disk utility.
7. Now, go to this page and download the latest version of CarbonCopyCloner:
www.bombich.com
CCC is FREE to download and use for 30 days -- this will cost you nothing.
8. Launch CCC and accept all the defaults for now.
Click through until you get to the main screen.
You will see "three boxes".
- Click on the left box and select your internal drive (the source)
- Click on the middle box and select the SSD (the target)
- IGNORE the box on the right, not needed (scheduling)
- Now, click the "clone" button and follow through.
9. CCC will take a while to "clone everything over".
10. When done, quit CCC. REBOOT, and IMMEDIATELY hold down the option key and KEEP HOLDING IT DOWN until the startup manager appears.
11. Do you see the external SSD as a boot drive choice? Good, click on it with the pointer and hit return.
12. Do you get a good boot from the SSD? When you get to the finder, it will look EXACTLY like the internal SSD. You have to go to "about this Mac" in the Apple menu to check to see which drive you're booted from.
13. IF you get a good boot, open up the startup disk preference pane and set the SSD to be the new boot drive.
14, Finally, reboot to check to see if the iMac will automatically "find and boot from" the SSD. Again, the boot should be much faster.