Yep, I tend to agree that a fully custom flow may be overkill for you. I think that using the standard approach, and just building onto it simply as you’ve suggested is perfectly fine. I mean, hey, why not ask the user for additional information after involking the standard user flow. That seems reasonable to me. You would simply then have to have a way of dealing with that data (or lack of it) in terms of validation and how those new fields interact with the users table/user registration.
Best of luck with your development!