When I added stm32 support I did not consider the inconvenience reusing the flash_offset setting, also used by esp32, could cause. This could be quite inconvenient for anyone developing an app meant to run on either device family.
I think changing the stm32 platform to use stm_flash_offset would be helpful for users, as they could configure the offset for both platforms independently and just run the respective flash tasks without passing an offset or changing mix.exs.