Hi AGWalsh:
You are welcome to post in our forum.
From your description, we understand that you want to create a dynamic drop-down form in Microsoft Forms. Based on my tests and investigations, this requirement is currently not possible in Forms, I'm very sorry for the inconvenience.
We also found a similar post but need to use Power App:Solved: Dynamic Drop Down List in Form - Power Platform Community (microsoft.com) You can refer to the suggestions in this document if it helps you.
However, we mainly support O365 products so that we may not be able to answer your questions about PowerApps.
If you have further questions, to avoid sharing any incorrect information from our side, it is also recommended to ask the question in the Power Apps Community - Power Platform Community (microsoft.com) for a better help. Since your question is relate to PowerApps, it is better suited to post in that community. And it has more experts in this area to help you.
Thank you for your understanding.
Best Regards,
Tin