In my opinion, there is absolutely no difference between Sign Up and Sign In. Both require you to sign in and both automatically generate a new user row if needed. It’s just two different names for something that does the exact same thing.
I recommend not using a form because you will end up with a second user row which will cause issues. Other than that, yes you would just control tab visibility based on if a user profile is completed or not.
But they do the same thing, correct? With any other website that I’ve interacted with, they won’t let you Sign In until you have Signed Up. A login attempt will be rejected until you have an account. With Glide you could click on either one as a New or Existing user and gain access into the app whether or not you have an account. To me Sign Up means onboarding and collecting information before signing in. With Glide, I think having two separate buttons is more about making the user feel less confused, but both buttons are functionally the same.
You can either do a detail screen with entry components linked to the same row, or you could have a button that leads to an edit screen to edit the user profile.