I think that’s a good argument for frequency goals, i.e. do more often not just do more quantity. That can be achieved with a custom aggregation method but I’m not sure that any of the fitbit measures would work even with a nonzero aggregation.
You might be better off creating a manual goal. Just give yourself a point for every day that you exercised, and don’t worry about the fitbit integration.
Set your slope to an achievable number of workouts per week. Probably not seven, so that you can catch up after the occasional day off!