Okay. The problem with your current method is it is very easily broken. As I mentioned earlier, all it would take is for a single row to be inserted or deleted in any sheet, and everything breaks.
I’m not sure if this could work for your use case, because I’m still not really understanding how the rows in your secondary sheets are added, but what you might be able to do is create a single screen onboarding for new users, then make them tap a button before they access the rest of the App. Then you could have an action attached to that button that adds a row for that user to each of your secondary sheets. At that point, you could set their email address immediately.
If you’re not sure what I mean, make a copy of the App below and study it. It demonstrates exactly what I am talking about.