Send Custom email template to custom email without TargetObject id

public class EmailClassUtility
{

    

    public static void sendmail()

    {

        messaging.SingleEmailMessage email = new Messaging.SingleEmailMessage();

        list<contact> con = [select id from contact where id ='0036F00002s555oQAA'];

        String []toAddresses = new String[]{'XXXXx@gmail.com'};

        EmailTemplate objEmailTemplate=[select id, name from EmailTemplate where Name='Custom template'];

        email.setTemplateId(objEmailTemplate.id); 

        email.setToAddresses(toAddresses); // custom email field

        email.setWhatId('a0B6F00001crms0UAA'); // custom object id for template

        email.setTreatTargetObjectAsRecipient(false); // not send email to contact

       email.setTargetObjectId(con[0].id);

        Messaging.sendEmail(New Messaging.SingleEmailMessage[]{email});

    }

    

}