Thanks - that does make sense and it is similar to the pattern I am attempting to implement. There is a difference however, this example appears to be quite different in AEM 6.1 where the submit is being handled in it's entirety.
From what I gather in 6.3, a custom submit action can be used (like I am using), but I don't think that is where the final redirect to the thank you page is executed (correct me if I am wrong), because I don't have access to the code where the final redirect takes place (unlike the stack overflow example) I cannot add to the request the attributes that I would later retrieve on the thank you page. I can only add them as parameters, which will be shown in the query string and will not meet my security requirements.
Can you confirm in AEM 6.3 where the actual redirect to the thank you page occurs ?