Those vendor libraries are super ugly and take the fun out of a project.. I absolutely hate the tedious "work" programming where you plug bits of pre-built technology together without actually doing much of anything on your own.
A while back I had an STM32F7 (ST cortex M7) board that I wrote some assembly code for. Getting correct clock signals and power to all the peripherals and bringing up RAM and configuring devices sure was complex (and it's not explained by the datasheet in a particularly straightforward way), but doing it that way is certainly enlightening and interesting, compared to fiddling with a vendor-supplied toolchain.
However... I would recommend not using a Cortex M device if you decide to go the ARM route. Pick a full classic-arm-ISA device so you can use the more human-oriented instruction set. You can stills witch to THUMB if you want, but you won't be forced to deal with its quirks and limitations.
A while back I had an STM32F7 (ST cortex M7) board that I wrote some assembly code for. Getting correct clock signals and power to all the peripherals and bringing up RAM and configuring devices sure was complex (and it's not explained by the datasheet in a particularly straightforward way), but doing it that way is certainly enlightening and interesting, compared to fiddling with a vendor-supplied toolchain.
However... I would recommend not using a Cortex M device if you decide to go the ARM route. Pick a full classic-arm-ISA device so you can use the more human-oriented instruction set. You can stills witch to THUMB if you want, but you won't be forced to deal with its quirks and limitations.