Obviously the final decision is yours alone. In fairness Greece has had a low level of infections but it is apparently rising slowly. In general I think Greece is fairly safe, but it does depend on your accommodation and holiday plans. Personally I wouldn’t go to a hotel because there will be many people there. It would be much easier to stay away from too many people in a holiday home or independent apartment. Where will you eat? Hotels restaurants may have lots of people around you, whereas eating in a holiday home would be much safer. Or even eating in a taverna outside, especially if it’s not crowded and near the sea.
All travel carries risk of the virus, including travel in this country. Has anyone seen the pictures of the beaches in Devon and Cornwall at the moment, or the packed streets in the little Cornish villages? Having said that, September will undoubtedly be a much quieter month.