Kumanichi reported on two sunset photography opportunities around the solstice, which lasts through December. One is Kumamoto Airport, where the setting sun aligns with the runway. The second is the Midorikawa estuary (which you should visit anyway - It’s quite natural, not concreted in like Shirakawa), where the setting sun silhouettes a torii gate on an islet known as Tahareshima (風流島). Two examples are at the link. If you catch anything good, please share with us. - William