This is good systems design. I've made a system with a similar design (RPI for display and UI, one STM32 for hardware control, another as watchdog) for an industrial marking system, and it's run almost daily for 3 years without any issues.
The great thing about the Pi in this case is that it's non-critical, and if it ever does die, you spend $35 and put in a new SD card.
The great thing about the Pi in this case is that it's non-critical, and if it ever does die, you spend $35 and put in a new SD card.