platform_specific_components VS stm32-bootloader

Not sure what STM32 CAN peripherals you are talking about (bxCAN vs FDCAN) but if you are using bxCAN, I recommend looking at UAVCAN's implementation: https://github.com/UAVCAN/platform_specific_components since it is very simple to use (even though it claimed works with libcanard stack only but should be fine for outside purposes). I think putting RTOS on it should be simple enough. For FDCAN, the only options right now are from ST HAL or ChibiOS AFAIK.

Here's a bootloader "library" that might be useful. You need to add your own code to get the actual firmware image from somewhere (UART, an SD card, or whatever), and then you just pass it off to the "library" for flashing.

Yes, unless you have something like an STM32 with a built-in bootloader. This can be used to update the regular bootloader. For example, this bootloader is capable of jumping to the built-in bootloader from software (as opposed to the "regular" method of pulling certain pins high or low at reset to cause the processor to run the built-in bootloader from ROM rather than the regular bootloader from flash).