Hi Erin Kimak,
Thanks for using Microsoft Q&A forum and posting your query.
I don't think you can decrypt (using REST API) the package content that was generated by ADF service in its activity payload. Your package will be automatically compressed and embedded in the activity payload and once embedded, you can only Download your package later for editing/viewing purposes from ADF Studio.
In order to view the package, you will have to first download the package and then use VS2019 to view the content/design of the package. For more information, please refer to this thread: Open a dtsx file in design view.
Hope this info helps.
Thank you