Hi Tim
I think you can solve this by putting an OnChange event handler on Reference Set Selector 1 which commits the object. Look in the properties window for Reference (Set) Selector 1 and you'll see an Events section, which will enable you to point to a microflow that will be executed when a change is made to the Reference (Set) Selector.
Hope that helps,
Mike
As Mike suggests, the OnChange workflow is what you are looking for. I think it might also be worth taking a look at the Questionnaire module from the app store https://appstore.home.mendix.com/link/app/234/Mendix/Questionnaire-(module)
This includes workflow to conditionally show questions based on the answers to previous questions, which sounds like what you are trying to achieve. Even if you don't use this module, it may give you some pointers to how to approach some of the challenges.