The ending point of the Slime anime in the manga can vary. Sometimes the anime might catch up to a certain point in the manga storyline, but it's not always a straightforward match. It could also stop before reaching a major plot development.
You can start from the chapter where the anime season 2 ended. Usually, you can find this information on manga forums or from fellow fans' discussions.
The Slime manga is mostly adhered to in the anime. There could be some tweaks in character expressions or certain scenes' sequencing, but these don't significantly change the essence of the story.