I have went trough both of the courses (I might have another quick check) :)
Unfortunately the credit data are not part of the Customer data model. It is life data, that change often and we need to get the most current value when the strategy is invoked.
I think the solution would be to get the credit balance before calling the container and then send the value with the request to the container, then use the data from the ContainerPayload.
However regarding the REST call.
It is only for inbound Container (to prevent system overload) and the REST call has to be parametrized. Is there any option/suggestion how to incorporate such a call to the strategy to get the response and then process the strategy?
I think it goes a bit against the phylosophy of Pega Marketing module to quickly process big amounts of data, but in case anyone was facing similar problem please share.
I'm not sure I understand what you mean by "Is there any option/suggestion how to incorporate such a call to the strategy to get the response and then process the strategy?"
The container request call to Pega Customer Decision Hub/Pega Marketing contains a JSON payload. The application is shipped with a default JSON structure but you can customize this during implementation to suite your needs.