To tackle this task, I would start by using Python's requests library to fetch the JSON data from the REST API. Once I have the data, I would use the pandas library to load the JSON into a DataFrame for easier manipulation and analysis. This allows me to clean and transform the data as needed, ensuring it's ready for visualization.

Next, I would focus on identifying the key metrics that need to be visualized. With these metrics in mind, I would use Python libraries such as Matplotlib or Seaborn to create visualizations. These libraries are great for generating a variety of plots and charts, which can help in understanding the data better.

Finally, I would integrate the visualizations into Power BI for more interactive and dynamic dashboards. Power BI's ability to handle real-time data and provide rich visualizations would enhance the overall presentation of the key metrics. Throughout this process, I would ensure that the data is accurately represented and insights are clearly communicated to stakeholders.