If first, skip next calls. If second, skip next call,...

matt-excerpt.php

<?php
$yoastdesc     = get_post_meta( get_the_ID(), '_yoast_wpseo_metadesc', true );
$excerptlength = get_theme_mod( 'excerpt_length', 25 );
//$excerpt       = get_the_excerpt(); //moved inside condition
//$content       = get_the_content(); //moved inside condition
$screenreader = '<a href="' . get_permalink() . '"><span class="screen-reader-text">' . get_the_title() . '</span>Read More &hellip;</a>';

$echoed_excerpt = '';

if ( ! empty( $yoastdesc ) ) {
	$trimyoast      = wp_trim_words( $yoastdesc, $excerptlength, $screenreader );
	$echoed_excerpt = $trimyoast;
} elseif ( has_excerpt() === true ) {
	// moved $excerpt here so it never gets called if($yoastdesc)
	$excerpt = get_the_excerpt();

	$trimexcerpt    = wp_trim_words( $excerpt, $excerptlength, $screenreader );
	$echoed_excerpt = strip_shortcodes( $trimexcerpt );
} else {
	// moved $content here so it never gets called if($yoastdesc) ||  has_excerpt() === true
	$content = get_the_content();

	$trimmed_content = wp_trim_words( $content, $excerptlength, $screenreader );
	$echoed_excerpt  = strip_shortcodes( $trimmed_content );
}

echo $echoed_excerpt;