The best place to start the Demon Slayer manga is right at the beginning. It lays out the background and hooks you into the story. And don't worry if it seems a bit slow at first; it builds up nicely as you go along.
I'd suggest starting at Chapter 95. It smoothly picks up where Season 3 leaves off and continues the exciting plot. There are new characters and challenges introduced that will keep you engaged.
It starts at a specific chapter, but I can't remember exactly. You might need to check online forums or manga-related websites for the precise details.
You could start from the first volume. That's usually the best place to get into the story and understand the characters and world-building from the beginning.