To me the onboarding MUST be done by a Nostr-only App Store.
I don't want to onboard to an app. I want to onboard to an ecosystem of apps and allow the user to choose.
User creation can be done by the store.
Imagine if nostr:nprofile1qqs83nn04fezvsu89p8xg7axjwye2u67errat3dx2um725fs7qnrqlgzqtdq0 had an onboarding screen that automatically installed Amber and logged into that for the user. Now you have a signer for all the other apps you may want to install.
The goal is to onboard to Nostr, not to any specific brand inside of Nostr.