Redirect to url after signup (from within rule?)

Oh, I see! Yes, the application would handle the redirect in that case instead of a rule.

You could alter your rule to instead add a flag in the ID token that will let your app know that the user requires onboarding:

function (user, context, callback) {
  var loginCount = context.stats && context.stats.loginsCount ? context.stats.loginsCount : 0;
  var namespace = "http://your-app-uri"; // a namespace is required for custom claims to avoid collision
  
  if (loginCount <= 1) {
    context.idToken[namespace + 'requiresOnboarding'] = true;
  }
  
  return callback(null, user, context);
}

Here is info about setting up a single callback URL in your app that handles redirects: