Set post as WP Front-Page

post-as-frontpage.php

<?php

add_filter( 'get_pages', 'add_posts_to_frontpage_dropdown', 20, 2 );
add_action( 'pre_get_posts', 'update_frontpage_post_query' );

function add_posts_to_frontpage_dropdown( $pages, $r ) {

	if ( array_key_exists( 'name', $r ) && 'page_on_front' == $r['name'] ) {
		$args  = [
			'post_type'      => 'post',
			'posts_per_page' => - 1,
			'post_status' 	 => 'publish',
		];
		$items = get_posts( $args );
		$pages = array_merge( $pages, $items );
	}

	return $pages;
}

function update_frontpage_post_query( $query ) {
	if ( ! $query->is_home() || ! $query->is_main_query() ) {
		return;
	}

	// get_option() does not work because of the built in filters
	global $wpdb;
	$row = $wpdb->get_row( $wpdb->prepare( "SELECT option_value FROM $wpdb->options WHERE option_name = %s LIMIT 1", 'page_on_front' ) );
	if ( ! is_object( $row ) ) {
		return;
	}

	$args = "p={$row->option_value}";
	$query->init();
	$query->parse_query( $args );
}