I think AVR is a good architecture to learn on, because the devices are far simpler than ARM or other more modern chips. The main concept to master as an embedded developer is operating a device by its registers. The common arduino device, ATMEGA328, has a total of 84 registers across all peripherals. A simple program to blink the LED is less than 10 lines total. The datasheet is tiny compared to most ARM devices, but is still a complete source of information.
There are lots of libraries online that demonstrate how to set up registers for various peripherals. Avrgcc is an open source toolchain, and avrdude can program devices running the arduino bootloader.
That was true when one had to write assembly code. Now we don't really have to any more, thanx to Arduino mainly (which bilds upon avrgcc and avrdude). It's easier to learn on the new platforms because you can use higher level programming languages like MicroPython, especially for easy stuff. I can do a blinky by writing three lines in the uPy REPL.
If the registers are abstracted away, you're left relying on other people's code. I suppose learning microcontrollers can mean different things to different people, but as for becoming a professional embedded developer, micropython and Arduino are dead ends. There is no transition to truly understanding the hardware.
We use MicroPython commercially so my perspective is different. :) I do agree with your intent though; just because you're using a high-level language it doesn't excuse you from learning how the hardware works.
However, if you do know the hardware, if you are familiar with what's going on under the hood, then MicroPython allows you to write the majority of your system significantly faster.
This is subject to change, as embeded developers now focus on writing lower or higher level code mainly in C/C++ as the industry migrates away from 8-bit MCUs to 32-bit ARM and RISC-V. Of course you're relying on other people's code, as you can't expect everyone to write their own RTOS and standard library and also achieve an acceptable time to market. Most embedded hardware products use an existing RTOS and build upon it. Understanding how the hardware works always helps, but most of the time you don't have to dig to register level to do that.
Micropython and Arduino are not dead ends but hobby grade tools.
There are lots of libraries online that demonstrate how to set up registers for various peripherals. Avrgcc is an open source toolchain, and avrdude can program devices running the arduino bootloader.