How to use variable from customizer.php in customizer.js

Use wp_add_inline_script() to export the variable from PHP to JS. For example, in my_styles_method assuming that your customizr.js script has the handle my-styles-customize-preview:

wp_add_inline_script(
    'my-styles-customize-preview',
    sprintf( 'var MyStylesSelector = %s;', wp_json_encode( $custom_css ) ), 
    'before' 
);

Then you can modify for JS to be:

wp.customize( 'wpt_heading_color', function( value ) {
    value.bind( function( newval ) {
        $( MyStylesSelector ).css( 'color', newval );
    });
});

Nevertheless, I suggest a better approach and that is to update the inline style element instead of setting the style for each element. For example:

wp.customize( 'wpt_heading_color', function( setting ) {
    setting.bind( function( value ) {
        $( '#custom-style-inline-css' ).text( MyStylesSelector + '{ color: ' + value + '}' );
    });
});

This is also a good candidate for using a custom selective refresh Partial that short-circuits the partial refresh request to the server and instead handles the rendering client-side by overriding the refresh method.