You have multiple options here.
1. Define post type archive slug on post type registration
By setting 'has_archive' => 'galleries'
you can define custom post type archive slug.
Check documentation.
Then you can delete your page “galleries” then add & customize the archive-post_type.php
2. Disable default archive for post type
Disable the archive by setting 'has_archive' => false
then keep the page for the post type archive.
3. Redirect archive requests to your page
You can permanent redirect default archive request to your page.
function archive_to_custom_archive() {
if( is_post_type_archive( 'post_slug' ) ) {
wp_redirect( home_url( '/galleries/' ), 301 );
exit();
}
}
add_action( 'template_redirect', 'archive_to_custom_archive' );
I will say the first method is good!