WordPress How do I pass a variable from one add_action to another?

I’m trying to add a custom field on the checkout form if a certain product category is in the basket and then on submission, check that field has been completed and add the result to the order details.

I have the form field working correctly but not the other actions and it is because the variable $course_in_cart is not being passed to the other actions. All actions work correctly if I remove the $course_in_cart test but it needs to be conditional and this is where I’m failing.

The first action checks if there is a courses category product in the cart, write sthe html and sets the variable to true.

// Set course in cart to false
$course_in_cart = false;

add_action( 'woocommerce_after_checkout_billing_form', 'digitalessence_check_course_category_in_cart' );
function digitalessence_check_course_category_in_cart($checkout) {
    global $course_in_cart;
    // Loop through all products in cart and if there is a course.
        foreach ( WC()->cart->get_cart() as $cart_item_key => $cart_item ) {
            // If Cart has a course category, set $cat_in_cart to true
            if ( has_term( 'courses', 'product_cat', $cart_item['product_id'] ) ) {
                $course_in_cart = true;
                break;
            }
    }
    // If there is a course in the cart, write HTML form field
    if ( $course_in_cart) {
           echo '<div id="ebike-question">';
           echo '  <div class="woocommerce-additional-fields__field-wrapper">';
             woocommerce_form_field( 'E-Bike__Present', array(
        'type'          => 'text',
        'class'         => array('E-Bike__Present'),
        'placeholder'   => __('Are you bringing an e-bike on the course?'),
        'required'  => true,
        'label'     => __('E-Bike', 'woocommerce'),
        ), $checkout->get_value( 'E-Bike__Present' ));
            echo '</div>';
           echo '  <div class="ebike-question-end-div"></div>';
           echo '</div>';      
    }
    }

The next action I want to perform is to check the status of $course_in_cart and if it is true, add a required field notice. This is where I fall down as it is alsways showing as false.

add_action('woocommerce_checkout_process', 'my_custom_checkout_field_process');
// Check if set, if its not set add an error.
function my_custom_checkout_field_process() {
    global $course_in_cart;
    if ( $course_in_cart) {
        if ( ! $_POST['E-Bike__Present'] )
        wc_add_notice( __( '<strong>For our legs, we need to know if you are bringing an e-bike!</strong>' ), 'error' );
        }
    else {
        wc_add_notice( __( '<strong>course in cart variable is showing as false</strong>' ), 'error' );
        
    }
}

I stripped everything right back to basics to ensure that passing variables from one function to another would work.

<?php
$course_in_cart = false;
declareVariable();
displayVariable();

function declareVariable() {
  global $course_in_cart;
  $course_in_cart = true;
}
function displayVariable() {
    global $course_in_cart;
    //echo $course_in_cart; // outputs 1
    if ( $course_in_cart) {
        echo "Its working. The variable is showing as: " . $course_in_cart;
    }
}
?>

And it does. I get “Its working. The variable is showing as: 1” in the browser.

Any and all help appreciated. I have searched other questions and haven’t found any similar so hope this isn’t a duplicate. Please berate me if it is! And I hope I’ve given enough information. If not…

Thank you.