Adding to the_content or a variable from within a shortcode function

I think, I would simply store all the definitions during the glossary_shortcode() execution in a global variable and serve them at the end of some hook like the_content.

function glossary_shortcode( $atts = array(), $shortcode_content = null ) {
    // Your code can be kept identical, just store the $glossary_term_object somewhere.

    global $glossaries;
    $glossaries = $glossaries ?? [];
    $glossaries[] = glossary_term_object;
}

add_filter( 'the_content', function ( $content ) {
 
    global $glossaries;
    if ( !empty( $glossaries ) ) {
        echo '<ul class="glossary">';

        foreach ( $glossaries as $glossary ) {
                printf( '<li><a href="%s" target="_blank">%s : %s</a></li>',
                        get_the_permalink( $glossary ),
                        $glossary->post_title,
                        $glossary->post_content
                );
        }

        echo '</ul>';
    }
 
    return $content;
}