You can create custom thank you pages in PageBuilder, and then have the survey point to that page. After you have built the thank you page in PageBuilder, navigate here:
  1. Content, Survey
  2. Edit the survey in question
  3. On Page 1. Identify the Survey, scroll to Step 11: Survey Submitted Page
  4. Click Select, and choose the PageBuilder page you created
  5. Click Insert
  6. Click Save on the survey