Java Full Stack is any day better. You need to learn a single language(in your case it's Java) and a framework(Springboot/Quarkus). Personally I would choose Spring boot. You already know react.js. So I guess a good project that includes microservice architecture, inter service communication, distributed logging and tracing, connection with some db, api gateway, service config, service discovery, etc. You can then deploy this to cloud, containerise the services and deploy using docker. At last if you need you can use Kubernetes as well