I'd say count yourself lucky that you worked on a practical, if indeed archaic example. My own experience was with the 68HC11 (which is far further out of production than the 8051). Although valid, I don't know that a completely simulator-oriented introduction to concepts like assembler can be as instructive as the literal hands-on architecture experience provided by even these older CPUs. In fact, though schools often focus on only one option, the advantage of learning the abstract alongside both the older Motorola or Intel implementations can be instructive. Having also used the STM32 development board during my education I would argue that the instruction set complexity has less direct applicability to an introductory assembler or architecture program. Have you considered RISC-V or were you just trying to get to anything more modern? The complexity of STM32, though certainly powerful, probably makes it less than ideal as an introduction to basic assembler concepts. (Of course, one thing an STM32 board could definitely do would be to run a virtual environment  for something like LC-3, another consideration for you) Good luck!

Hey I really appreciate the time you put in to go through this.

I was just trying to move beyond classwork, and with practically no guidance. I wanted to explore hardware design, and this I saw as window.

Firmware was not really the focus, and haven't been able to put time into it either.

From what I could understand STM32 seemed like a popular choice for General purpose microcontrollers.

The availability to the part also played a role in the decision.

Popular is an understatement and when it comes to availability you made a GREAT selection: the STM32 is, for the power, cheap and ubiquitous! One application to consider for the STM32 would be real-time operating systems as software like FreeRTOS is free (beer/gratis) and fairly permissive (free speech/libre). From there you would likely gain a solid footing to decide where you want to go next. Warning: I'm biased in part as this was my own approach when learning the STM32 platform...

Good luck!

Great to see this helping out someone, considering this is close to heart and I'm not getting time to really work on this, and take this where I wanted it to.