No, it is not possible to manually trigger a ""Thank You"" email for respondents who have already completed the survey.
However as a workaround, you can send survey response to respondents from Response viewer by clicking on Response id > Click on Email icon on top-right corner.
Screenshot: https://monosnap.com/direct/G3PRDwyZKOQjT43fmUYNdZ824C5jsK
Helpfile: https://www.questionpro.com/help/response-viewer.html