Buschy, the variation is huge even when they come from the same country.
Can I suggest something for you to try on your blending journey? Use one SO and roast it to various profiles (you seem to have got the hang of that already ;), then blend say 25% of the darker roast with your lightest roast. Its simple, can add some great complexity, and help you understand your SOs even better, before making further adjustments.
Hope thats of some use!