Security - even just understanding the various protocols and limitations of things like JWT would be useful.
Automated Testing - I'm a Test-Driven Development advocate, but I also know that's rarely easy to incorporate into existing codebases. However, the ability to write good, small, focused, developer tests is a very useful skill. And not just because of the tests, but often because it helps you implement functionality in smaller steps.
HTTP API Design - Especially since it involves designing protocols without you realizing you're designing a protocol.
Without more details, you’ll probably get folks recommending whatever language they prefer - to do whatever it is they’re doing on the backend. It’s not necessarily what you care about.
And to be a “senior” developer implies you’re wanting to make use of this at some company.
Again, what is the expectation of the role? Writing services in Go or Java to handle FANG scale traffic is different than building a backend workflow that migrates data from one system into another at some recurring cadence.
One book that I think has a lot of value for different backend problems is “Designing Data Intensive Applications” by Martin Kleppman. It’ll help you build a foundation of the different varieties of architecture and problem spaces in this area and a toolkit on how to think about them.
Having said that a great deal of being senior is possessing people skills - communication, being able to give feedback etc. That’s what I would focus on - learning tech should be the east part!
