Suggestions workflow user registration in form tool

Hi,
I have build a wizard tool to calculate water use and warming cost in showers with nice graphs on the confirmation page.
see https://papagreen.org/tools/bereken-je-voordeel-van-een-warme-terug-winning-douche/

I now study whats the best way to get people more involved by making in account with a personal page where several results of these tools can be shown.
Think Buddypress.

So first of all, whats in your opinion the best way to get people registrated?
With the https://www.gravityforms.com/add-ons/user-registration/ I know its possible.

Questions:
what works best out of a conversion point of view?

  1. Start with asking there email/username/password
  2. Or ask these in step 3
  3. ask just email and password? And later ask more questins.

Because there is no official demo video of /add-ons/user-registration/ Iam not sure if it can this:

  • already registered users dont see the -required- fields for registration.
  • make the registration in a way its leas intrusive and more user friendly.
  • choose for double opt-in or single opt-in? Can a user start the forms, the finish the form without checking email and confirming the email link for double opt-in?
  • other ways of confirmation?

All examples of live sites are welcome so i can study UX and best practices

nobody? is this not a normal usecase?