learndash_course_completion_url

 <?php
/**
 * Example usage for learndash_course_completion_url filter.
 */
add_filter(
	'learndash_course_completion_url',
	function( $link, $course_id ) {
		// You can change the link here
		if ( $course_id == 123 ) {
			$link = get_permalink( 456 );
		} elseif ( $course_id == 456 ) {
			$link = get_permalink( 789 );
		}

		// Always return $link
		return $link;
	},
	5,
	2
);

add_filter(
	'learndash_course_completion_url',
	function( $link, $course_id ) {

		// Change $course_id to the completed course
		if ( $course_id == 33749 ) {

			// Change to the next couese ID
			$link = get_permalink( XXX );

		} else {

			// Change to a default redirect or you can add more course redirects later
			// Or don't change the value of $link to let the user return to the completed course URL
			$link = get_home_url();
		}

		return $link;
	},
	5,
	2
);