In fiction, no single book will make a career… Write in series and if possible, write multiple series for the same readers. Promotion needs to be continual. Effective promotion options will vary among genres/sub-sub-sub-genres
Watch that genre/sub-sub-sub-genre for changes and evolution. Keep an eye on how covers are changing over time and make sure you re-cover as needed.
In some genres/sub-sub-sub-genres, things can change fast. Recently, I watched one Indie author radically transform an entire genre/sub-sub-sub-genre in less than two months. So, do keep close eye on things 😉
The beauty of fiction is that it is entertainment, not a problem’s solution, so fans will buy over and over if they like the genre/sub-sub-sub-genre and author.