How do I show a link or ‘Read More’ button on a custom field excerpt when it is less than the word limit

You can count the number of words in your text to check if it’s greater than 15 using the str_word_count php function, and if not return your read more link also

So i would modify your custom field excerpt thus:

function custom_field_excerpt() {
    global $post;
    $text = get_field('description');
    if ('' != $text) {
        $text = strip_shortcodes($text);
        $text = apply_filters('the_content', $text);
        $text = str_replace(']]>', ']]>', $text);
        $excerpt_length = 15;
        $excerpt_more = apply_filters('excerpt_more', ' ');
        $text = wp_trim_words($text, $excerpt_length, $excerpt_more);
    }

    if (str_word_count($text) > 15)
        return apply_filters('the_excerpt', $text);

    return apply_filters('the_excerpt', $text) . "<a class="moretag" href="https://wordpress.stackexchange.com/questions/251744/get_permalink($post->ID)"><br>...Read More</a>"
}