Insert After Paragraph

/**
 * Featured Image, Classic Editor
 *
 */
function be_single_featured_image_classic( $content ) {

	// Only run on the main post
	global $wp_query;
	if( 0 !== $wp_query->current_post )
		return $content;

	// Only run on classic editor, see next function for block editor
	if( has_block( 'core/paragraph', $content ) )
		return $content;

	$image = get_the_post_thumbnail( get_the_ID(), 'large' );
	if( !empty( $image ) )
		$content = ea_insert_after_paragraph( $image, 2, $content );
	return $content;

}
add_filter( 'the_content', 'be_single_featured_image_classic', 5 );

/**
 * Featured Image, Block Editor
 *
 */
function be_single_featured_image_block( $content, $block ) {

	// Only run on the main post
	global $wp_query;
	if( 0 !== $wp_query->current_post )
		return $content;

	if( 'core/paragraph' !== $block['blockName'] )
		return $content;

	global $be_current_paragraph;
	if( empty( $be_current_paragraph ) )
		$be_current_paragraph = 1;
	else
		$be_current_paragraph++;

	if( 2 !== $be_current_paragraph )
		return $content;

	$image = get_the_post_thumbnail( get_the_ID(), 'large' );
	if( !empty( $image ) )
		$content .= '<div class="wp-block-image">' . $image . '</div>';
	return $content;
}
add_filter( 'render_block', 'be_single_featured_image_block', 10, 2 );

/**
 * Insert after Paragraph
 * When running on the_content, use priority > 20 so it doesn't affect oEmbed
 *
 * @param string $insertion
 * @param int $paragraph_id
 * @param string $content
 * @return string $modified_content
 */
function ea_insert_after_paragraph( $insertion, $paragraph_id, $content, $include_at_end = true ) {
	$closing_p = '</p>';
	$paragraphs = explode( $closing_p, wpautop( $content ) );
	foreach ($paragraphs as $index => $paragraph) {

		if ( trim( $paragraph ) ) {
			$paragraphs[$index] .= $closing_p;
		}

		if ( $paragraph_id == $index + 1 ) {
			$paragraphs[$index] .= $insertion;
			$inserted = true;
		}
	}

	if( ! $inserted && $include_at_end )
		$paragraphs[] = $insertion;

	return implode( '', $paragraphs );
}