Add back the WYSIWYG content editor to the WordPress blog page and display the content on the front end

functions.php

<?php
/**
 * Add the WYSIWYG editor back to the blog page
 *
 * @param WP_Post $post The post object
 */
function cameronjonesweb_fix_no_editor_on_blog_page( $post ) {
	if( $post->ID === get_option( 'page_for_posts' ) ) {
		add_post_type_support( 'page', 'editor' );
	}
}

add_action( 'edit_form_after_title', 'cameronjonesweb_fix_no_editor_on_blog_page' );

/**
 * Display the blog page content before the posts listings
 */
function cameronjonesweb_blog_page_content() {
	if ( is_home() ) {
		$content = apply_filters( 'the_content', get_post_field( 'post_content', get_option( 'page_for_posts' ) ) );
		echo wp_kses_post( $content );
	}
}

add_action( 'loop_start', 'cameronjonesweb_blog_page_content' );

gutenberg.php

<?php
/**
 * Allow the blog page to be edited using the Gutenberg plugin on WordPress 4.9.8
 *
 * @param bool    $can_edit Whether the post can be edited or not.
 * @param WP_Post $post The post object
 * @return bool
 */
function cameronjonesweb_fix_no_gutenberg_on_blog_page( $can_edit, $post ) {
	if( $post->ID === intval( get_option( 'page_for_posts' ) ) ) {
		$can_edit = true;
	}
	return $can_edit;
}

add_filter( 'gutenberg_can_edit_post', 'cameronjonesweb_fix_no_gutenberg_on_blog_page' );
add_filter( 'use_block_editor_for_post', 'cameronjonesweb_fix_no_gutenberg_on_blog_page' );

only-first-blog-page.php

<?php
/**
 * Display the blog page content before the posts listings only on the first page
 */
function cameronjonesweb_blog_page_content() {
	if ( is_home() && ! is_paged() ) {
		$content = apply_filters( 'the_content', get_post_field( 'post_content', get_option( 'page_for_posts' ) ) );
		echo wp_kses_post( $content );
	}
}

add_action( 'loop_start', 'cameronjonesweb_blog_page_content' );