2-way encryption of url parameters

Question

My client uses a 3rd party vendor to store contacts and send promotional emails. All of the emails drive the user to various landing pages on their website. occasionally there is a need for personalization where the landing page may display the user's name, email address and/or other saved preferences. In the emails, we have the ability to merge fields, which I am currently using to pass their email address (encrypted) through the url to the landing page (ex www.example.com/landing-page/?UID=NGNVdGM2T3AvSo00UWkyQnUyeUNxczRRaXV2cHprdzRNUXV6S2ujoEZYakNteTAzRWZuekR4RmFReStXckE9PQ==).

If the page requires more of the user's info, the email address is used as the primary key in the vendor's API. So I may make a request to an endpoint such as (www.example-vendor-api.com/ACCOUNT_NUMBER/member) with a payload of { email: user@some-company.com } and the public and private API keys passed as Authentication header.

Right now, I have a custom field in the vendor database called uid. Any time someone completes a form and is added to that database, I encrypt their email address which is then stored in that column.

I know very little about this so I'm trying to find out if my encryption/decryption method offers adequate security or if a scenario such as this requires an entire library such as Pixelfck/SymmetricEncryption (this was recommended to me and looks fairly easy to use).

Anyway, I'm using PHP's openssl_* functions with aes-256-gcm cipher with a static key stored on our server and a pseudo-randomly generated IV. here's the PHP I got largely from this post.

function encrypt($encryptionKey, $data) {
    $iv = openssl_random_pseudo_bytes(openssl_cipher_iv_length('aes-256-gcm'));
    $encrypted = openssl_encrypt($data, 'aes-256-gcm', $encryptionKey, OPENSSL_RAW_DATA, $iv, $tag);
    return bin2hex($iv . $tag . $encrypted);
}

function decrypt($encryptionKey, $data) {
    $c = hex2bin($data);
    $ivlen = openssl_cipher_iv_length($cipher="AES-256-GCM");
    $iv = substr($c, 0, $ivlen);
    $tag = substr($c, $ivlen, $taglen=16);
    $ciphertext_raw = substr($c, $ivlen+$taglen);
    return openssl_decrypt($ciphertext_raw, 'aes-256-gcm', $encryptionKey, OPENSSL_RAW_DATA, $iv, $tag);
}

If anyone could comment on the level of security this provides in relation to the use case and/or if I should be using an existing library, I'd be most appreciative.