You have the instincts of a true critic. Your habit of flipping to pages in the middle is what so many critics do, I wrote about in a Medium post on something called "the Page 69 Test."

I flip less because authors are supposed to start with a bang than because it can take them a few chapters to find their footing: to find their voice, to get on top of the plot, etc. The test has value no matter why you do it.

You're right: You do have a huge in judging covers from having been an art director and having an understanding of what makes a cover "pop." The IngramSpark story I linked to has good tips for those who lack your expertise.