Much gratitude for your guidance, all. You have convinced me to install my own RAM. My friend Jude, who has effected this operation on her own iMac, said she would hold my hand (after grounding herself, to be sure, Trev).

If anyone else is thinking of doing this to their own late 2015 27-inch iMac, but hesitating for reasons similar to my own, there is a helpful video here.