限制来自ACF字段的字符数输出，并读取更多链接到另一个ACF值

I am trying to pull content from an ACF (Advanced Custom Field) value and trim it by character count. I want to append the truncated string with a 'Read More' text that uses an href value from another ACF field.

<?php
    $content = get_field('app_description');
    //^get $content from ACF field, no problem
    echo mb_strimwidth($content, 0, 306, "... <a class=\"f-size14 external\" href=\"' . get_field('app_link') . '\">Read More</a>");
?>

When I do this, it appears correct until you click on the "Read More" link. It outputs "https://demosite.com/page/%20.%20get_field(app_link"

How can I trim a field to a character count, and use another value / variable for the href?

I have also tried adding this to my functions.php

function get_prod_desc( $count, $rmtext ) {
    $link = get_field('app_link');
    $excerpt = get_field('app_description');
    $excerpt = strip_tags($excerpt);
    $excerpt = substr($excerpt, 0, $count);
    $excerpt = substr($excerpt, 0, strripos($excerpt, " "));
    $excerpt = '<p>'.$excerpt.'... <a href="'.$link.'" class="f-size14     external">'. $rmtext .'</a></p>';
    return $excerpt;
}

and calling it in my template like this

echo get_prod_desc(300, 'view more');

The results are very inconsistent in terms of actual character count, and it adds the 'view more' to content blocks under 300.