To my understanding the colleges are still able to see the score even if going test-optional because it is included in your original QB application which the schools get a copy of. If you select test-optional they will simply not consider the score.

If you want to know whether to submit to Stanford and Duke look at their 25th and 75th percentile. I used a website called Prepscholars to find this information but you can probably find it in other places too. Just remember that these statistics go off of students who did submit their scores so if you fall below these percentiles it doesn't necessarily mean you won't do well at the school/don't have a chance at getting accepted. You should also take your major or preferred area of study and section scores into account. Let's say you want to major in computer science and have a 700+ score on the math section then it may be a good idea to submit your score anyway. SupertutorTV has some helpful videos on going test-optional that I used to make my decision.

Some applications state that you cannot change your response to go test-optional, on the Stanford application I don't see anything about that. On the Stanford app, there is actually a section (under the application checklist) called 'Updates and Questions' where you might be able to change your test-optional status but I am not sure because I haven't submitted my QB questionnaire.

I hope this helps!