I think one of the easier ways to do right by your cats is to always have two or more. When we got Gentle Ben, Max Power suddenly reverse-aged by like five years.
I've had a number of cats over the years, but the best experience for me and for the cats has been with pairs from the same litter or who grow up together. It makes a huge difference when they have company.
You have to know your cat well enough to know which is right for them.
Our next door neighbor had a single older cat. The cat had a weight problem so they got a book, "How to exercise your cat". As I recall, the opening sentence was, "If you have 2 cats, you don't need this book."