If you're comfortable working with the Topics or Weeks format, there's an experimental feature called Course relative dates (admin shortname enablecourserelativedates, found under Site admin > Development > Experimental settings), which should enable you to set a number of days between activities, based on a participant's enrolment start date. It hasn't been fully implemented (it's still an experimental feature), but it may be useful.
Moodle Docs on the topic: https://docs.moodle.org/402/en/Course_relative_dates
Moodle Docs on the topic: https://docs.moodle.org/402/en/Course_relative_dates