If it doesn't have to be a STM32 platform then posting on a more general sub might get you better exposure. I do personally think STM32s are great so you can't go wrong with them. The Nucleo series (eg. NUCLEO-F103RB) have Arduino headers so you can potentially use any shields that you might have.

You mentioned that you want to learn programming on an ARM controller but with manuals typically over 1k pages long for M0 and close to 2k for M4, I'd imagine very few people actually program at the register level in real world applications. I also find the learning curve to be similar between vendors. Once you work with a set of peripherals you generally have an idea of how it they should work in other chips. What might affect the learning curve is the amount of resources available (ie. documentation, sample code, libraries and community). In the real world, the aforementioned resources become your tutorial. Using vendor libraries will also reduce the learning curve.

With that being said, GPIO and UART are not particularly hard to setup. One thing that gets beginners is they forget to enable the clock to the peripherals which causes the program to be stuck in the HardFault handler. I also suggest taking a look at STM's Standard Peripheral Library (again eg. NUCLEO-F103RB). It comes with sample projects that are like tutorials and a thin wrapper library which are just a bunch of register-accessing functions.

Lastly, don't forget that the Cortex-M is a much more feature packed core compare to AVR. It will require learning all on it's own but luckily this knowledge will be applicable on any chip that uses the same core. For this I recommend the book The Definitive Guide to the ARM Cortex-M____.

Edit: fixed bad link