Whenever members of this forum are asked what saddle they use for long distance, there seems to be a preference for leather saddles instead of synthetic. It doesn't mean that a synthetic saddle will not work for you, however. Having said this, if your preference is to have your handlebar lower than your saddle, there are saddle models that work better than others regardless of material. For example, the Fizik Aliante (synthetic) and Brooks B17 (leather) are supposed to be for cyclists with little flexibility which require their handlebars about level with their handlebars. If you're overall comfortable with your handlebar at 6.5 cm below saddle without lower back discomfort on long rides, you are quite flexible. So, it's possible that your current saddle shape and preferred handlebar height are not compatible causing the chafing and saddle sores issues. At any rate, just something to look into.