In this BookBub Partners post, How Successful Authors Use Social Media: 24 Content Ideas, Leila Hirschfeld pulls together two dozen ideas that are working well for other authors.
Take a look through and see if there's an idea or two you haven't tried that resonates.
Unexpected and interesting ones that popped for me included:
#4. Show props from a scene in a book,
like in this example by Cheryl Bradshaw
#15. Create little games for fans,
like in this example by Sylvia Day