What's the best resource to learn those things in practice? Other than _just trying_ yourself? I'm a senior dev in another area who wants to get into the backend development, but unsurprisingly has limited time to spend on learning a completely new thing,

There are some books out there like “Distributed Systems” by Tanenbaum. Then there are the various company publications like “The SRE Book” by google and eng blogs.

You could also contribute to an open source project like kubernetes or postgres to get your feet wet.

Like all things, the best way to get better is to do it.