Wordpress functions.php conditional include another functions file

In my wordpress functions.php I have calls to include other function files. For example like so:

include(functions/articles.php);
include(functions/custompost.php);

Within those files I have all my functions that pertain to specific pages on the site.

What I want to do is this:

//Articles custom functions
function articles_fn() {
 if ( is_page_template( 'page_templates/articles.php' ) ) {
    
        include(functions/articles.php);

    }
}
add_action( 'init', 'articles_fn');


//CustomPostType custom functions
function cpt_fn() {
 if ( is_page_template( 'page_templates/custompost.php' ) ) {
    
        include(functions/custompost.php);

    }
}
add_action( 'init', 'cpt_fn');

...but this doesn't work. Any help is hugely appreciated.

Thank you