I think finding a process that can create the flowchart based on the input on your form fields, might be very difficult.  But if your users are willing to use a process for clicking or drag-dropping to create the flowchat, then there appears to be a lot of options available out there from a Google search for "JQuery Flowchart".  You could potentially include one of those on your form (it'll have to be Classic Designer to make use of JQuery code).  I can't speak to any of these individual tools, I haven't used any of them.

The tricky part will be getting it to persist beyond submission.  One way to do that, is to export the base64 code of the image and store it in a multi-line field before submission.  Later on, you can reload the base64 to display what happened on prior submissions.  This is fairly easy to do if you are using the canvas object as it has built-in functions for working with the base64 code.  Note at that point though, it is a static image, the edits made previously are part of the image.  This kind of thing I have done several times.  This post is an example of some code that I wrote that works with the canvas and includes storing it as base64 and reloading it later from the base64.  This code is actually for the modern designer rather than the classic designer (which was more complicated to set up than it would be in the classic designer - the classic designer could work with a single canvas, but in the modern designer, I had to use two, a hidden one and a visible one and sync the changes between them).  This code just puts an X on the image where you click, it doesn't include functionality for other shapes or to drag/move shapes you already have on the image, but I think it's a decent demonstration of a few concepts (such as drawing on the canvas, saving the canvas to base64, and loading that base64 back into the canvas later).