Modify RSS – remove image and add text

Note that the_permalink_rss() and the_excerpt_rss() do echo the output, not return it.

With your current snippet, replace the_excerpt_rss() with:

apply_filters( 'the_excerpt_rss', get_the_excerpt() );

and replace the_permalink_rss() with:

esc_url( apply_filters( 'the_permalink_rss', get_permalink() ) );

You could also use:

<content:encoded><![CDATA[
    <?php  the_excerpt_rss(); ?> 
    <a href="https://wordpress.stackexchange.com/questions/245310/<?php the_permalink_rss(); ?>">
        <?php esc_html_e( 'Read more', 'mydomain' );?>
    </a>
    <?php esc_html_e( 'at', 'mydomain' );?>
    <?php the_permalink_rss(); ?>
?>]]></content:encoded>

where you strip the RSS excerpt through the the_excerpt_rss filter or use the above approach to get the excerpt output to strip it.

You could also try using printf with:

<content:encoded><![CDATA[%s <a href="https://wordpress.stackexchange.com/questions/245310/%s">%s</a> %s %s]]></content:encoded>

etc.