usually with STM32 you need to use --connect-under-reset, are you doing that?
are you trying to flash known-good firmware, or are you trying to flash your WIP Rust code?
I would recommend starting with a known-good setup and only changing one variable at once. Debugging is hard enough, you don't want to be debugging multiple things at once.
and, please report the commands you're running and your output in a gist. if you don't know what you're looking for, you're not going to be relaying useful info for those that do.