Try with below code for email template Recipient:

Xml code:

<record id="define_email_template" model="mail.template">

            <field name="model_id" ref="model_x_name"/>

            <field name="email_from">{{ user.email_formatted }}</field>

            <field name="body_html" type="html">

                <p>Dear <t t-out="ctx.get('Recipient')"/>,

                </p>

                # Rest contain

            </field>

        </record>

Python code:

def custom_method(self):

template = self.env.ref('define_email_template')

context = {}

         if template:

context.update({

    'Recipient': user.name or None, # Replace placeholder with user's name

})

template.write({

    'email_to': email,

})

template.with_context(**context).send_mail(self.id, force_send=True)

I figured out the issue. The reason it appeared as test was literally me making the appointment on preview using the company login credentials. I enabled the sign in button on the website, created a test account with my personal email and made an appointment. The email sent to my email now correctly addresses the name of the attendee.