You shouldn't be doing long web forms before creating an account anyway.
So in the email you can include a link to optional account (username/password) creation.
And if you aren't sending an email, then the user has clearly decided not to create an account.
If you demand lots of information that should be clear right away.
People shouldn't be making long forms part of account creation unless absolutely necessary; in which case it should already be obvious to the user that it is.
If it's somehow necessary but not obvious, you can put up a friendly warning. Maybe something like, "Step 1/12" or "expected registration time 15min."
/s, but only slightly.
You're right, of course. The answer is the one nobody wants to hear: you can tell users what you'll want from them, then ask to create an account, and then ask them to do the things you outlined before registration.
As for multi-page forms, they make sense if you target non-JavaScript use, but if your site is already an SPA, you might as well present the form in full, and conditionally disable parts that don't apply based on earlier inputs. This is the way to improve over the paper UX.