odoo-survey-addons/survey_input_template_custom/templates/sfinished.xml

<?xml version="1.0" encoding="utf-8"?>
<odoo>

  <template id="sfinished" inherit_id="survey.sfinished">
    <h1 position="replace">
      <t t-if="survey.thank_you_title">
        <h1 t-esc="survey.thank_you_title"/>
      </t>
      <t t-else="">
        <h1>Thank you!</h1>
      </t>
    </h1>
  </template>

</odoo>