Filter the_excerpt to show content in a more ideal way. Notes here: http://stevengliebe.com/2015/02/26/excerpts-and-the-more-tag-in-wordpress-themes/

excerpt-filter.php

<?php

/**
 * Filter the_excerpt to show content in a more ideal way.
 *
 * The way content is shown depends on the priority of a situation:
 *
 * 1. the_content with "Read More" link if the "More" tag is used
 * 2. Manual excerpt with "Read More" link if excerpt is manually entered
 * 3. the_content in its entirety if less than 200 words (nicer than automatic excerpt)
 * 4. Automatic excerpt with "Read More" link if no better situation
 *
 * This is based on the handling of excerpts in churchthemes.com themes with improvements inspired
 * by Devin Price and commenters: http://wptheming.com/2015/01/excerpt-versus-content-for-archives/
 *
 * @global object $post
 * @param string $excerpt Excerpt from get_the_excerpt()
 * @return string Modified excerpt
 */
function themeslug_the_excerpt( $excerpt ) {

	global $post;

	$show_excerpt = false;

	// "Read More" text
	/* translators: %s is post title (hidden, for screen readers) */
	$more_text = sprintf(
		__( 'Read More %s', 'themeslug' ),
		the_title( '<span class="screen-reader-text">&mdash; ', '</span>', false )
	);

	// 1. the_content with "Read More" link if the "More" tag is used, but not password-protected
	// This is first because it is as intentional as manual excerpt but manual excerpt could have other purpose (ie. meta description)
	if ( strpos( $post->post_content, '<!--more-->' ) && ! post_password_required() ) {
		the_content( $more_text );
	}

	// 2. Manual excerpt with "Read More" link if excerpt is manually provided
	elseif ( has_excerpt() ) {
		$show_excerpt = true;
	}

	// 3. the_content in its entirety if less than 200 words, but not password-protected (nicer than automatic excerpt)
	// A nice-looking fallback for when they do not use "more" or manual excerpt, or if only published a media embed
	elseif ( str_word_count( $post->post_content ) < 200 && ! post_password_required() ) {
		the_content();
	}

	// 4. Automatic excerpt if no better situation
	else {
		$show_excerpt = true;
	}

	// Show excerpt with "Read More"
	if ( $excerpt && $show_excerpt ) {

		?>

			<?php
			// Manual or automatic excerpt
			echo $excerpt;
			?>

			<p>
				<?php
				// "Read More" linked
				printf(
					'<a href="%1$s" class="more-link" rel="bookmark">%2$s</a>',
					esc_url( get_permalink() ),
					$more_text
				);
				?>
			</p>

		<?php

	}

}
add_filter( 'the_excerpt', 'themeslug_the_excerpt' );