You could use SharePoint Designer to add two new Finish buttons to newForm.aspx.
Detailed steps:https://sharepoint.stackexchange.com/questions/227917/sharepoint-2013-ootb-survey-redirect
In order to distinguish the two buttons, I used different names.
Code for your reference:button-code.txt
Then we could display different finish buttons according to the selection status of the Want to add more question.
Change button code for your reference:code.txt
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Code to change Finsh button redirect page:28672-code.txt
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