Hi,

Override the action_quotation_send function and pass the required mail template with condition of selected quotation template.

def action_quotation_send(self):
   self.ensure_one()
   if self.sale_order_template_id.id == 'condition':
       template_id = self._find_mail_template()# select the required mail template id
       lang = self.env.context.get('lang')
       template = self.env['mail.template'].browse(template_id)
       template.attachment_ids.unlink()
       if template.lang:
           lang = template._render_lang(self.ids)[self.id]
       ctx = {
           'default_model': 'sale.order',
           'default_res_id': self.ids[0],
           'default_use_template': bool(template_id),
           'default_template_id': template_id,
           'default_composition_mode': 'comment',
           'mark_so_as_sent': True,
           'custom_layout': "mail.mail_notification_paynow",
           'proforma': self.env.context.get('proforma', False),
           'force_email': True,
           'model_description': self.with_context(lang=lang).type_name,
           'default_attachment_name': [(6, 0, self.designer_attachment_ids.ids)]
       }
       return {
           'type': 'ir.actions.act_window',
           'view_mode': 'form',
           'res_model': 'mail.compose.message',
           'views': [(False, 'form')],
           'view_id': False,
           'target': 'new',
           'context': ctx,
       }

Regards