We are in a similar market, town of 9,000 with the closest shop being about 30 miles away. Sundays can be hit or miss. Our loyalty account system shows that most of our regulars stop in during the week, but we see an uptick of folks "just passing through" on Sunday. I think that we sell about another $800-1000 per month than we would if we were closed on Sundays
Like me, you are probably one of the biggest towns in your area. Lots of people that live in the surrounding very small towns come into town on Saturday and Sunday to hit up Walmart and the grocery stores so it might be good for you to open up on Sundays.
Also, a bit off topic, but what are your thoughts on opening at noon ans staying till 8:00 a day or two per week? Closing at 6 prevents many of the local "professionals" from being able to get in to see you.