Nice, but I suggest to use the "name" label for the field, "username" is midleading since Nostr has not usernames in the usual meaning of centralized platforms.

In Nstart I use "(nick)name", I think it is a good way to concisely explain this.

I would also add a contextual help explaining the matter, this is the introductory text in Nstart: "The name is not a unique username, we can have as many Jacks we want! Feel free to use your real name or a nickname; you can always change it later. But remember: online privacy matters, don't share sensitive data."