How to add custom HTML tags in the visual mode via keyboard only?

if you are using classic TinyMCE editor then perhaps you can modify it add custom styles.

Following code may help you create visual buttons for adding inline styles.

// Add Formats Dropdown Menu To MCE
if ( ! function_exists( 'mk_style_select' ) ) {
    function mk_style_select( $buttons ) {
        array_push( $buttons, 'styleselect' );
        return $buttons;
    }
}
add_filter( 'mce_buttons', 'mk_style_select' );

// Add new styles to the TinyMCE "formats" menu dropdown
if ( ! function_exists( 'mk_cusotm_styles_dropdown' ) ) {
    function mk_cusotm_styles_dropdown( $settings ) {

        // Create array of new styles
        $new_styles = array(
            array(
                'title' => __( 'Custom Styles', 'pixel-text' ),
                'items' => array(
                    array(
                        'title'     => __('Light','pixel_framework'),
                        'inline'    => 'span',
                        'classes'   => 'light',
                    ),
                    array(
                        'title'     => __('Regular','pixel_framework'),
                        'inline'    => 'span',
                        'classes'   => 'regular',
                    ),
                    array(
                        'title'     => __('Medium','pixel_framework'),
                        'inline'    => 'span',
                        'classes'   => 'medium',
                    ),
                    array(
                        'title'     => __('Bold','pixel_framework'),
                        'inline'    => 'span',
                        'classes'   => 'bold',
                    ),
                    
                    
                ),
            ),
        );

        // Merge old & new styles
        $settings['style_formats_merge'] = true;

        // Add new styles
        $settings['style_formats'] = json_encode( $new_styles );

        // Return New Settings
        return $settings;

    }
}
add_filter( 'tiny_mce_before_init', 'mk_cusotm_styles_dropdown' );