I think building your own Rails app from scratch is a great way to go if you have something that you’ll find fun to build, it can be a bit of a slog if it’s not something you are excited about.
I think there’s no way to learn about defining scope other then to try a project and then see what turns out to be ambitious, if it’s for your own thing, then you can structure it how you want and learn what’s hard by doing things. I think I’d try to build one concrete feature at a time, rather than spend a lot of time worrying about long term architecture.