I would choose (1), since you want to use that certificate for Roundcube/webmail. This should automatically secure the webmail for you.
Then, you will also need to manually configure Postfix to use the certificate.
You can use https://ssl-config.mozilla.org/#server=postfix&version=3.4.8&config=intermediate&openssl=1.1.1d&guideline=5.4 to guide you.
The certificate path is /etc/letsencrypt/live/mail.2e0epv.com/fullchain.pem and the private key path is /etc/letsencrypt/live/mail.2e0epv.com/privkey.pem.