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What year is your iMac?
If you have USB, just add an EXTERNAL USB3 SSD and set it up to be the boot drive.

The Mac will run MUCH faster, and you'll never even notice that you're "booted externally".

You DON'T need to pay anyone to do this.
You can do it yourself, easily.
 
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But the OP specifically said he wants an "internal" SSD. That can be done, but certainly not "easily."
 
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