Which tech stack should a student currently choose for SaaS development?

I don't think there is necessarily a 'correct' stack to learn, I feel like understanding what is required to build an E2E solution (frontend, api, services, model) irrespective of any specific framework is important for any junior engineer. Once you have these fundamentals you will have a much better idea about what you need to do to solve any problem. It's so overwhelming for a junior engineer (or any engineer) to navigate all the technology available to them so make sure that you get started and get over the initial hurdle - this is the hardest part!