If you don't like the eyes, I'd avoid some older Key/Visual Arts series, then, like Kanon. Those big eyes are prevalent in most Key/Visual Arts works (even Angel Beats has relatively big eyes for anime, but a bit on the small side for Key/Visual Arts works. Some really popular '90s series have those big eyes as well, like Slayers.
Thing is, most of those girls are the same age! The only ones that are not are Nagisa and Tomoyo, and they're only about one year apart in age from the other girls, Nagisa being older and Tomoyo being younger. They're all in their last two years of high school, and, according to various studies, most women stop growing in height around age 16, and the Clannad main girls are all around that age, so it makes sense that they're all looking about the same age. especially since many older individuals don't see much physical difference between a 17/18 year old and a 15/16 year old
As for "same-face" syndrome, that's rampant in many anime to begin with, you just don't realize just how rampant it is until you realize it exists. I do agree that it's hard to differentiate most of the girls if you were to only see their eyes, and the only way to tell them apart at times is the hairstyles. If you're only half-watching, the names won't stick because, like you've stated, the girls' faces look too damn similar.