Focus on C and follow up with an understanding of the underlying assembler code. Haven't played with Rust for about 1 year, so I don't how how mature it is these days.
Also have an idea of how to debug the code. Debugging on embedded is very different to debugging a web/js app.
Low-level systems programming is hard and can be a bit boring at times. I agree that debugging is very different from debugging on higher level systems. Most of the time an oscilloscope or an LED is the best debugging tool. Also, in well written low level code you tend to have more code making sure that everything is okay than you have actually doing stuff, which can be tedious. It takes a certain kind of personality to enjoy it.
 I have the LDD and https://www.amazon.com/Linux-Kernel-Development-Robert-Love book. I got confused about the authors name :)
The author Robert Love has done a tremendous job on Linux Kernel Development book. Every aspiring "In-betweeners and Low-Low Level engineers"  should read this book. It's awwwwespiring.
And please read LDD too!