Reserving certain usernames/making them unavailable to public

So, I'm playing around with this code snippet with the help of ChatGPT.

Goal: To make certain usernames unavailable to the public
For example, let's say you're running a Notion community and you wanna keep the @Notion (and other brand) username unavailable to the public.

I tested the code in FluentSnippets, and it's working.

The only issue is: I was able to pass through the signup form, even a verification code was sent to my email, and the error message was shown when inserting the verification code. I'd like to do it in a way that the user should not be able to go through the signup page and the error message should be shown on the signup form itself, before sending the verification code.

Any coding expert would like to try debugging this issue?

/**
 * Block exact-match reserved usernames during FluentCommunity registration
 * and all other WordPress registration flows.
 * Prevents sending verification codes to blocked usernames.
 */

// List of blocked usernames (lowercased)
function fecx_get_blocked_usernames() {
    return array_map('strtolower', array(
        'notion','asana', 'trello'
    ));
}

function fecx_is_blocked_username($username) {
    return in_array(strtolower(trim($username)), fecx_get_blocked_usernames(), true);
}

/**
 * 🧩 1. FluentCommunity pre-verification validation
 * Prevents the email from being sent if username is blocked.
 */
add_filter('fluent_community/validate_registration_data', function ($errors, $data) {
    if (!empty($data['username']) && fecx_is_blocked_username($data['username'])) {
        $errors['username'] = __('This username is not allowed. Please choose another.');
    }
    return $errors;
}, 10, 2);

/**
 * 🧩 2. WordPress core registration fallback hooks (for safety)
 */
add_filter('registration_errors', function ($errors, $sanitized_user_login, $user_email) {
    if (fecx_is_blocked_username($sanitized_user_login)) {
        $errors->add('username_reserved', __('This username is not allowed. Please choose another.'));
    }
    return $errors;
}, 10, 3);

add_action('register_post', function ($user_login, $user_email, $errors) {
    if (fecx_is_blocked_username($user_login)) {
        $errors->add('username_reserved', __('This username is not allowed. Please choose another.'));
    }
}, 10, 3);

add_action('user_profile_update_errors', function ($errors, $update, $user) {
    if (isset($user->user_login) && fecx_is_blocked_username($user->user_login)) {
        $errors->add('username_reserved', __('This username is reserved and cannot be used.'));
    }
}, 10, 3);

add_action('user_register', function ($user_id) {
    $user = get_userdata($user_id);
    if ($user && fecx_is_blocked_username($user->user_login)) {
        wp_delete_user($user_id);
        if (defined('WP_DEBUG') && WP_DEBUG) {
            error_log("Blocked username registration removed: {$user->user_login}");
        }
    }
});