Unable to import WXR via PHP on plugin activation

I'm setting up a large collection of sites for my non-profits chapters. They all need to start with a bit of boilerplate content and I'm hoping to automate the import process. However, I can't for the life of me get the importer to work programmatically. It seems I can't get WP_Import to load.

To simplify debugging, I moved just the import portion into its own plugin. It fails at line 42 (Checking for WP_Import and dying if it doesn't exist), which is consistent with my difficult getting ahold of WP_IMPORT.

<?php
/*
Plugin Name: WXR Import on Activation
Description: Automatically imports a WXR file into WordPress when the plugin is activated.
Version: 1.9
Author: Troutoil
*/

// Hook to run on plugin activation
register_activation_hook(__FILE__, 'import_wxr_on_activation');

function import_wxr_on_activation() {
    // Path to the WXR file
    $wxr_file_path = plugin_dir_path(__FILE__) . 'boilerplate.xml';

    // Check if the WXR file exists
    if (!file_exists($wxr_file_path)) {
        error_log('WXR file not found at: ' . $wxr_file_path);
        return;
    }

    // Ensure the WordPress Importer plugin is installed and activated
    if (!function_exists('is_plugin_active')) {
        require_once ABSPATH . 'wp-admin/includes/plugin.php';
    }

    $importer_plugin_slug = 'wordpress-importer/wordpress-importer.php';

    if (!is_plugin_active($importer_plugin_slug)) {
        wp_die('The WordPress Importer plugin must be installed and activated to run this script.');
    }

    // Load the WordPress Importer plugin directly
    $importer_path = WP_PLUGIN_DIR . '/wordpress-importer/wordpress-importer.php';
    if (!file_exists($importer_path)) {
        wp_die('The WordPress Importer plugin files are missing.');
    }
    require_once $importer_path;

    // Ensure the necessary class exists
    if (!class_exists('WP_Import')) {
        wp_die('Failed to load the WordPress Importer classes.');
    }

    // Run the importer directly
    $importer = new WP_Import();
    $importer->fetch_attachments = true; // Enable attachment import

    try {
        ob_start();
        $importer->import($wxr_file_path);
        $output = ob_get_clean();

        // Log the output for debugging
        error_log('WXR Import Output: ' . $output);

        // Set a success flag for admin notice
        update_option('wxr_import_success', true);
    } catch (Exception $e) {
        error_log('WXR Import Exception: ' . $e->getMessage());
    }
}

// Admin notice to inform if the import was successful
add_action('admin_notices', function() {
    if (get_option('wxr_import_success', false)) {
        echo '<div class="notice notice-success is-dismissible">
                <p>WXR file has been successfully imported.</p>
              </div>';
        delete_option('wxr_import_success');
    }
});

Any ideas would be greatly appreciated. I've been pulling my hair out for days!