I don't see anything here indicative of replacement or excess wear. Most of the members covered it, but what you're looking for is cracks, thickness of the disc that are approaching and past the min thickness (looks like you did a great job confirming that) and disc thickness variation, so if you measure and 4 different positions along the disc (12 oclock, 3, 6, 9) is there an uneven wear or runout?
you can also check your pad thickness to ensure you have appropriate amount of pad thickness (life). At 20k mi street miles, i don't expect there to be anything crazy. have you noticed excess braking related noises or loss in stopping performance?