You can't go wrong following this advice. Please do.

I agree. The best questions will come when you look to answer your own passionate questions to build a project you’ve dreamt.

This was my approach to learning Ruby on Rails when I joined a startup using it 2 years ago. This is also my standard approach learning anything new since I started programming.

I would like to augment this advice with the very helpful but rarely mentioned step of actually reading through all the docs eventually and learning best practices from research on blog posts, open source projects, and other internet communities. This is especially important when learning a technology in the absence of a mentor/senior developer.

In the past, I’ve been burned thinking I built up a serviceable knowledge through learning as I go on my own projects. This approach is a great way that falls in line with the fact that a lot of new technologies are still just the same old fundamentals, but it’s important to recognize that true skill in a specific technology requires that extra, quite laborious, step.

I agree with you that building the thing you want is the best way to learn.

However, for an "opinionated" framework like Rails, it's really best if you read through the official (brief, and IMHO excellent) guides so you can get a feel for the basic opinions baked into Rails.

https://guides.rubyonrails.org/

Otherwise you're going to be fighting the conventions the entire way.