Arduino is a good background for the general idea but I would suggest getting a “real” micro controller dev board. Or you can use the Arduino board without the arduinoisms.

One thing that’s important with embedded is never assume anything “should” do what it’s supposed to. There are a lot of dependencies especially as you go up the micro controller ladder. I would suggest looking at an 8bit pic or maybe a stm32L. Find some example code and really make sure you know exactly what each piece of code is doing. Learn a tiny bit of assembly. (I’m no expert but from more experienced people I’ve learned writing more than a few lines is very rare but reading it is more common)

For what it’s worth I was in your shoes a year ago and I’m currently at an embedded internship.