Fastest, easiest, cheapest, safest way to add an SSD:
Buy a USB3 external SSD, plug it in, initialize it to Mac OS extended with journaling enabled, and set it up to become your "external booter".
It will run at 85% (or better) the speed an internally-installed SSD will.
And... you don't have to pry it open and risk breaking something inside.
A Samsung t5 would do the job very well.
You can velcro it to the back of the iMac's stand, up and out-of-the-way.
Either install a new version of the OS onto it, or use a cloning app (CarbonCopyCloner or SuperDuper) to clone over your existing installation (the one that's on the internal drive).
Again -- fastest, easiest, cheapest... and safest.