There are a few copies of ‘Read More’ in the files. I think the one you want is in template-parts/entry/entry-summary-footer.php.
However if you look at the code that includes this in content-blog-horizontal:
if ( sinatra_option( 'blog_horizontal_read_more' ) ) {
get_template_part( 'template-parts/entry/entry-summary-footer' );
}
there’s already a config option ‘Show Read More Button’ to turn this off. I don’t know Sinatra to say where to find this in the admin site, but it should be there.
Regardless it’s risky to actually edit the theme because any changes you make would get lost when the theme updates. It’s safer to use filters / pluggables you set up in a plugin instead (as you’ve done for the excerpt function?) or a child theme to make changes.