There are a couple of different ways to do this.
1. You can add the More tag to each post where you want it to split.
2. You can use a plugin like this one to automatically add a More tag for you. (I haven’t used that specific plugin, it’s just one example you could try.)
3. You can make a change in your child theme to automatically display a post excerpt on the blog. Since you have a child theme set up already, you might prefer this route. Here are the steps:
(i) Copy content.php from the parent theme into your child if it’s not already.
(ii) In your child theme’s copy of content.php, find this line (it’s line 27 if you haven’t already made other modifications)
<?php if ( is_search() ) : // Only display Excerpts for Search ?>
(iii) In your child theme’s copy of content.php, change that line to:
<?php if ( is_search() || is_home() ) : // Only display Excerpts for Search and Blog ?>
Let me know how it goes.