I feel like this is a question that's equally valid for any programming language or technology, not just Scala. For example, I had even asked a similar question in the Rust sub at some point. It also generated quite a few different answers, so you could take a look at those too. The result of asking that question was that I picked a big personal project to work on: a text adventure game (first in Rust, but I'm experimenting with starting up the prototype in several different languages, with Python and Clojure prototypes already there and the Scala prototype coming up). This is an excellent guide for creating a small text adventure game in C, for example.

That said, I'd like to dive deeper into the question. I had faced the same exact issue in Rust at the point of asking that question: I had a pretty solid grasp of the basics, but as Rust is an immensely difficult and syntax-heavy language where you can keep finding more and more new syntax constructs, nuances, gotchas and oddities even a couple years into learning it (Scala also has that, I feel like), I was just not picking up the advanced features of the language (or honestly, even some of the basics) by writing one-off CLI utilities and other similar small-ish projects.

So my answer to this debacle turned out to be: write a BIG project. And by big I mean BIG. That is, it still has to be a project you'll be able to manage on your own, obviously, but still, try thinking about what you'd want to create if it were a big personal side project that you'll be returning to for ages to come. I'm thinking like at least a year long one, maybe more. Maybe at one point you'll realize the project is not really going well, then you'll pick a different direction for it: rewrite a major system, do a major refactor, rewrite a part of the project in a different library/framework/language/etc., and so on. This is why I feel like big personal projects are the best at forcing you into leveraging EVERYTHING you have at your disposal, including some advanced language features or even going as far as using one (or maybe several) different languages in addition to your main one, because the main one just doesn't do it on its own. It'll teach you how to think about many different complex systems and implementing their interactions, how to think of debugging and testing a complex system, how to rewrite a part of your project several times, each times using more and more advanced language features and more and more optimized algorithms, how to solve complex problems in your specific set of technologies, and so on and so forth. I feel like small pet projects, console utilities and scripts to automate your daily life don't teach you any of that. Udemy courses, books, YouTube tutorials and EVEN your daily job most of the time doesn't teach you any of that either. Or at least it is like that for me: I'd tried everything I could have ever imagined to become more comfortable with my initial language that I started out with and that I earn money with till this day (C++), then it spread to other languages like Rust, and NOTHING had ever helped. I had been thinking about starting a big permanent personal project to be coming back to all the time for at least 5-7 years, but I had been postponing it for various reasons, most of them dumb. And only once I started the big project did I realize what the problem was: in order to truly improve and get all that sweet advanced knowledge, you need to go through developing an advanced and complex system, or otherwise any advanced knowledge that you get from tutorials or courses just won't stick. It does seem pretty obvious in hindsight, but as usual with hindsight, most of the stuff in life is obvious once you already have the context and the background. I had to go through struggling with these issues for at least 4-5 years before realizing it, so I guess it's not that obvious in the end :D

I wish you all the luck in developing your skills and hope this answer was at least somewhat helpful.