Using the SIS imports has worked well for us. I generally only get a couple of requests a week after the initial uptake so isn't too time consuming at this point.
Parents are given an info flyer when they are enrolling their child, encouraging them to request an account. Sometimes this is only followed through on when the teachers suggest it after lots of questions are coming up etc. Either the parents or teacher emails me the details and I set it up. That relationship and explanation by the teacher is definitely the key factor for us.
Self-enrolling is not really a viable option for us at this point for a number of reasons. Maybe in the future.
I have been using an ID system of OBS(student ID) to keep it simple and consistent. They are sent a welcome email with their log in details and a video showing them how to access and use the account both on computer and mobile apps.
Some of the parents love it and access frequently. Some have never access the account despite requesting.
We don't have anything else to compare it to so can't answer the questions around improved uptake.