I have started watching and following this amazing youtube channel (Brian's Channel) that focuses on playing solo sandbox campaigns with various classic RPG systems.
The notes for his solo campaigns are so detailed and thoughtful. Definitely great treasures for solo gamers!!
http://youtube.com/post/UgkxJkcZjeSzbBiETBrOFcQ6HJJeT03mJ01y?si=yJYsjflWJMNe_3WH