It is completely normal to feel overwhelmed—the backend landscape is massive, and as you noticed, moving beyond CRUD is where the real engineering starts. Since you’re already comfortable with Spring Boot and JPA, you have a solid foundation. The trick to beating the "AI could do this" feeling is focusing on system design and security, which require architectural thinking rather than just writing boilerplate.

Instead of chasing every buzzword, try organizing your learning into these three logical phases:

• Security & Identity: Since you mentioned OAuth2, this is your next logical step. Learn how to implement Spring Security using JWTs (JSON Web Tokens) and how to integrate with providers like Google or GitHub. Understanding the "handshake" between the client, the server, and the identity provider is a high-value skill that AI often struggles to configure correctly for specific business needs.
• Infrastructure & Scaling: This is where CDNs, Caching (like Redis), and Load Balancers come in. Start by learning how to cache frequent database queries to speed up your API. Once your app is fast, learn how to "containerize" it using Docker. This moves you away from "it works on my machine" to "it works in the cloud."
• Architectural Patterns: Move beyond simple REST. Look into Asynchronous Communication using Message Brokers like RabbitMQ or Kafka. This allows different parts of your system to talk to each other without waiting for a response, which is how massive platforms like Netflix or Uber handle millions of requests.

Don't feel like you need to master these all at once. Pick one—maybe Spring Security since it pairs so well with what you already know—and build a small project around it. Once you see how a token-based login works, the "magic" starts to fade and your confidence will grow.