No matter which front/back-end framework you choose, SQL is a must. It's the main powerhouse when it comes to data management. There's hardly any company in the world which doesn't use SQL. So there are high chances that you will be interacting with relational databases while working on real world projects. My suggestion therefore would be to focus on SQL as much as possible. Any relational database (MSSQL/MySQL/Postgres) will work.
About web developments frameworks, I believe in having more than 1 programming language in your skill set. Sure it's easy to learn only 1 and use it for both frontend and backend which is the case of MEAN/MERN stack. But main advantage of being a polyglot is that you get the chance enhancing your own thinking process. Every language comes up with it's own set of pros and cons. Implementation of problem X in Python may vary in Java or Go. By interacting with multiple languages, you learn new was of observing and attacking a problem.
Then there's also a possibility that some other framework may work much better than node/JS for the development of backend for reasons related with nature of the business problem, scalability, performance and technical debt. On smaller scale, these may not matter that much but as application starts to grow, they can become a huge bottleneck.
Finally, having diverse tool set increases your chances of getting hiring too simply because you know more stuff as compare to those who're stuck with single ecosystem. It can also send a signal to recruiters that you're flexible enough to learn things which are outside of your comfort zone (some disagree with this point).
In the case of web app development, I would suggest you to learn JS for frontend and Python/Java for backend. Whatever combination you decide in the end, SQL is a must.