We would like to use a two steps login/registration where users would first enter only their username and validate, with a "remember me" checkbox and a link to account creation form.

If the user is known, he's asked for his password before (with a password forgotten link) being redirected to the final service.

If he's not, a message is shown explaining that the user doesn't have an account + a link to the registration form.

The registration form could be handled by HelloId or be a simple link to a form from another tool