You would start by installing the Server Automation REST API service.
Create Baseline manually or via Platform REST API.
Prepare XML and post to create SA Plan with Baseline and Targets via Server Automation REST API
Post action via Server Automation REST API… this part is not exactly intuitive… use API to export plan to XML, then use API and post exported XML to create plan action
Make sure you have the latest version of Server Automation installed in your environment.
I have worked a lot with the SA API for the past two years and all you mentioned is possible. I have a very basic example on how to use the API in my GitHub repo, feel free to take a look and use it as needed.