Custom copyright in GeneratePress whilst still allowing the customiser control to be used

functions.php

<?php
// Add this to your functions.php of your child theme
function generatepress_custom_copyright() {
    
    $copyrightCustomizer = get_theme_mod('generate_copyright');
    if ( !empty($copyrightCustomizer) ) {
        $date = date('Y');
        $copyrightCustomizer = str_replace( '%current_year%', $date, $copyrightCustomizer );
        $copyright = str_replace( '%copy%', '&copy;', $copyrightCustomizer );
        // change this to what you want after the copyright
        $copyright .= ' | Website built on the <a href="https:$copyrightCustomizer//generatepress.com/">GeneratePress Theme</a>';
    } else {
        // change this for default copyright if customizer is empty
        $copyright = sprintf( '<span class="copyright">&copy; %1$s</span> &bull; <a href="%2$s" target="_blank" itemprop="url">%3$s</a>',
            date( 'Y' ),
            esc_url( 'https://generatepress.com' ),
            __( 'GeneratePress','generatepress' )
        );
    }
    return $copyright;
}
add_filter( 'generate_copyright','generatepress_custom_copyright' );