Do you still think it's worthwhile to build a portfolio? I've read that people will do that when hunting for their first job, but afterwards it's all about job experience to them. Are we aiming to pass the personal project off as work experience? I think that'd be a tough sell.
It’s an opportunity to build skills you wouldn’t normally get a chance to at your job. It can be a way to showcase skills you aren’t fully able to showcase sufficiently via your resume. It’s a way to gain confidence in your skills. It’s a way to continue practicing, that’s why I specifically say it should be an enjoyable project. If practicing is a burden, what is the point?
The goal of creating a personal project is not necessarily building a portfolio. That is one aspect of it that can be useful and I think will be useful for you over time.
We would be using the project as a way to build confidence and get experience with designing a system, managing a long term project (feature prioritization, maintainability), and better understanding the project lifecycle as a whole, versus in a typical work environment you don’t usually get that holistic view of a project.