I am interested in this too. I am trying actually to start embedded development for a specific platform, but it's hard to understand where is a good starting point even as experienced software developer.
As someone who has quite a bit of experience as a systems level engineer, who has written code for custom hardware, but is not a firmware engineer (in other words, grain of salt here): Knowing C (or C++) well enough to be able to write threaded code is super important, for not all but many jobs. (Or at least know the concepts, eg the different kinds of atomics.) Knowing I²C or similar is a good idea. Knowing or dipping your toes into DSP can be helpful. Know at least mean squared error, and how to do a bandpass. And bonus points (though very few people in the embedded space know it) is learn Rust. It will teach you good programming practices that most firmware engineers do not know. Many of them are stuck bug testing run time errors day in and day out, instead of letting the compiler do it for them. (Again, 2 cents on all of this. Best to get actual requirements from job posts.)