Hi Albert

There is only fixed date available on a program level rather than a course set level.

If you could put users in a audience through a rule then you can use audience based visibility with a date rule so the courses are only visible to members of the audience.

Alternatively you could split each course set into its own program and make the second program only able to be enrolled in by people who have completed the first program through audiences. You can add an Available From date for the second program.

Regards