Handling in-app registration is complicated

You can't simply treat it as a form with username/email/password because of the potential for abuse, so you need to implement a captcha (which we do, CF Turnstile or hCaptcha) and so you need to use a webview, and return the oAuth token if successful, but if they are already logged in, you have to show a screen to logout

Not to brag but most fedi projects are either: easy to use, beautiful or feature packed. Most have one, some have two, but all 3? 😎