I'm not sure specifically what the'sama sama letter day story' is about as it's not a widely known common story. It could be a very personal or a story from a specific local or cultural context.
The Kaguya-sama manga is indeed finished. It ended after a successful run, leaving fans with memories of an engaging and enjoyable story. The final chapters tied up loose ends and delivered a conclusive ending.
Yes. The Kaguya-sama manga has reached its conclusion. It provided a satisfying ending for fans. The creator wrapped up the plotlines and characters' arcs neatly.
Yes, there might be such fanfics out there. You can try searching on fanfic platforms like Archive of Our Own or Fanfiction.net. They have a wide range of fan - created works and there's a good chance you'll find one related to Harry being a fan of Jashin Sama.