Why does it need a user account at all? You say it's to uphold privacy best practices, but I don't know what data it's collecting from my phone. I would hope none. But if no data is collected, I don't see why any authentication is required.
It's been easiest for us to do this user account creation using out-of-the-box Google Authentication, so that's why we went down this route. Hope that makes sense!
First, you should ask for an account creation only when the user have already started. As I said, I didn’t even see if it was interesting at all, because of the login wall.
Second, if there are no sensitive information collected, there is no need of using a mountain to hit the nail. A pseudo will be enough. No email needed. Not sure if it even need a password... You store the pseudo locally and you are done.
(I agree I’m a little bit oversimplifying, but you see the point)