I get your point and yes, a basic sign-up or login flow can take a few hours to code using a framework. But when you start adding features like OTPs, social login with account linking, conditional logic, data validation, 3rd party widgets to verify identity or link bank accounts, automations to send a welcome email, store the data in your CRM, etc. And as you grow, more things come up and that's when this starts to get messy and most of the time you regret reinventing the wheel.
I think only the welcome email is really valuable to have from all the things you listed. And if you can't handle few ifs within your sign in logic... good luck with your business logic.
I can attest to the pains of working with a complex onboarding flow... At my job, we have about 30 different paths the user could take depending on their usecase. And it's not modeled as 1 30-prong fork that everyone routes through, it's forks that lead back to other steps, skip steps, get data from elsewhere, etc.
The difficulty is that it's typically the product and marketing teams who need to understand the flows the most, in order to understand the conversion metrics and how to optimize, yet they have almost no visibility into the actual state logic that dictates the flow in code. Giving them more control over control flow is a huge win
