I have Strava for something else that I did but no longer use/d it. It is, however, linked to my Fitbit account. When I have activities like running or walking, it also saves with Strava. Since I can't get Fitbit totals by activity, I can use Strava.
Interval workouts are not uploading to Strava. There is a GPS option in Intervals though it's not defaulted like in running or walking. I use the GPS and do some of my interval runs using the Interval feature. I've given all the permissions I think I can and I'm not sure if the problem is on Fitbit's side or Strava's. On the off chance that somebody uses Strava and the Interval workout with Fitbit, have you been able to make it work? If so, how?
