Interesting, you know more about it than I do, I suppose. Do you know of any source for Jim discussing how/why he started Garfield?

https://en.wikipedia.org/wiki/Garfield#History is probably a good start.

Wikipedia is a bit coy and trying to be neutral. But even just from there you can see that the author decided to make strips about cats, because Snoopy had already cornered the dog market.