WP: Replace initial format select with custom

gistfile1.txt

/*
 * TinyMCE Custom Styles
 * https://codex.wordpress.org/TinyMCE_Custom_Styles
*/

// Callback function to insert 'styleselect' into the $buttons array
function my_mce_buttons( $buttons ) {
	unset( $buttons[0] );
	array_unshift( $buttons, 'styleselect' );
	return $buttons;
}
// Register our callback to the appropriate filter
add_filter( 'mce_buttons', 'my_mce_buttons' );

// Callback function to filter the MCE settings
function my_mce_before_init_insert_formats( $init_array ) {
	// Define the style_formats array
	$style_formats = array(
		// Each array child is a format with it's own settings
		array(
			'title' => 'H1',
			'block' => 'h1',
		),
		array(
			'title' => 'H3',
			'block' => 'h2',
		),
		array(
			'title' => 'H3',
			'block' => 'h3',
		),
		array(
			'title' => 'H4',
			'block' => 'h4',
		),
		array(
			'title'   => 'Paragraph 1 (18px)',
			'block'   => 'p',
			'classes' => 'p1',
		),
		array(
			'title'   => 'Paragraph 2 (16px)',
			'block'   => 'p',
			'classes' => 'p2',
		),
		array(
			'title'   => 'Paragraph 3 (14px)',
			'block'   => 'p',
			'classes' => 'p3',
		),
	);
	// Insert the array, JSON ENCODED, into 'style_formats'
	$init_array['style_formats'] = wp_json_encode( $style_formats );

	return $init_array;
}
// Attach callback to 'tiny_mce_before_init'
add_filter( 'tiny_mce_before_init', 'my_mce_before_init_insert_formats' );