Although it is not currently possible to set up the post submission Thank You screen using Survey123 Connect, we do have it listed as an Enhancement to be considered in a future release. I will add your comments to our existing record, and in the meantime, please take advantage of the ability to achieve this using the web designer.
I am also in need of being able to edit the post submission screen for a project for informal appraisal appeals (land records ). Connect has so much additional functionality over the webforms that I really want to take advantage of. Is it possible to edit this anywhere in any of the files that are auto generated in the C:\Users\username\ArcGIS\My Survey Designs\projectname?
We've made an enhancement in 3.3 release to allow editing the thank you screen content of Survey123 Connect surveys directly in the Design page of Survey123 website, just as you edit it for the surveys published from the website. Please be noticed after editing and publishing the survey on the website, you have to use 3.3 or higher version of the web app to see the result.
The 3.3 release is planned before the Dev Summit, please stay tuned with our release blog and doc to find more details.
Thanks again for all your feedback!