What data source should be used for a FlexCard design when the required SOAP endpoint is not ready yet?

The optimal choice for a FlexCard design when the required SOAP endpoint is not ready yet is Integration Procedures. This is mainly because Integration Procedures are designed to execute complex business logic, gather data from various sources, and perform multiple processes in a single call. They offer flexibility in integrating with both Salesforce and external systems, thus allowing developers to prepare for the eventual integration of SOAP endpoints while still being able to build and test their FlexCard.

By utilizing Integration Procedures, you can simulate responses, handle data transformations, and manage various data sources effectively. This capability is particularly beneficial when working in a scenario where the SOAP endpoint is tardy, as it allows for the development of the user interface to proceed without having to depend on the availability of that specific service. This means that while waiting for the SOAP endpoint to be ready, you can still continue building and refining your FlexCard solution.

Other options, while useful in different contexts, either do not provide the necessary functionality for the integration needs or are more suited for different types of interactions or data manipulations within Salesforce.