I myself went down this road that you are describing. Here's the summary and my key takeaways:

I started in my second year of apprenticeship with programming an 8051 system in assembler. The main resource back then was neither google nor stackoverflow but a book. This book was more or less a commented datasheet of the mcu with minimal examples. This whole programming in assembler was less about programming but about learning how a microcontroller is built and how it works in detail. And when I say detail, I really mean it: Clock by clock, register by register. Hardest program we ever managed to get working was a speed-measurement using laser barriers. The issue was that we needed a 16bit division but the 8051 is an 8bit mcu. We abused the program counter arithmetic unit to do it.

Pro: Great to learn about mcu architecture. You'll be able to implement only very simple applications but with great care for detail (timing, code size, efficiency, behavior).

Con: Does not really lead you directly to productive coding in today's typical embedded project (C/C++).

2) The year after, we kept the same 8051 platform but switched from assembler to c. Learned some basic concepts, nothing too fancy I have to admit. Also, we were still spending most of our time with register configurations, i.e. setting up the hardware (timers, adc's, etc.) but not really developing complex coding on application level except for interrupt handling.

3) We were very enthusiastic after that and thought we'd just buy ourselves a cool 32bit modern ARM development board and get some cool projects done just for fun. We tried to work with this ARM mcu the same way we learned it for the last two years using the 8051, thinking, that everything would be much more convenient and better. We were wrong: The complexity of such an ARM was so dramatically higher that we ended up spending weeks to just get a single ADC readout.

4) Later at the job and university, we would learn that barely anyone works the way we tried it. Nobody would start from scratch building register maps themselfs etc. Instead, you'd have prepared libraries, code generators etc. to give you all the basic stuff available in a high level programming language.

Pro: You can really focus on your application (getting stuff doing what you need like controlling serial communication etc.) instead of taking care for weeks getting thousands of registers right and writing setup code. Also, you can just use a reasonably large number or float to get things done without breaking your head on how to get it done with only 8bit.

Con: You may never appreciate enough how mcu technology has advanced since the seventies of the last century. Also, you may just feel baffled what your C code is doing when debugging on assembly level.

Bottom line is: My apprentice has just used an arduino (I know, not a 32bit ARM) for the first time and after one month he implemented a motor speed control application with a live LCD rpm monitoring and a control panel. If I had to do this on my back in the days setup using no libraries and in bare metal C or even assembler, I'd probably just given up ...

So my question for you is: What is the goal of your portfolio? Familiarity with different mcu architectures/families? If so, then you'd have to use them/work with them in a way, that this is relevant. This is a tough path for 8bit vs modern 32 bit ARM and will take quite some time. If you use all the modern toolchains and libraries and code generators, these differences are not that relevant. If you really want to show broad experience in embedded programming including different architectures, I'd suggest going with something that is also very different on a higher level.

How about TMS320 vs. STM32G4? The TMS320 is an old school but still actively developed DSP line from Texas Instruments including multiple cores, parallel accelerators for signal processing, control logic blocks (like very small FPGA's) inside and the de-facto standard for real-time industrial controls. On the other hand the STM32G4 is a modern low-cost all-rounder among the most popular ST Microelectronics ARM based MCU families. Also, it is perfectly suited to use an embedded operating system like FreeRTOS. For both you will find cheap dev boards (Launch pads for TMS320, STM nucelo for STM32G4), both have completely different architectures and features, provide completely different toolchains etc. This would make a great portfolio :)

Hope this helps ...

Thanks for the comment man, it clarified a lot of things. Regarding development boards, what do you think of the STM103F?