So I opened the mower again to verify it's an STM chip, with a suspicion of it being a less known STM32 F402. And that was confirmed by the markings on it's top after removing the orange sticker. It did say ST 402 but there was no easier way to be sure. I read that it's compatible with F401 chips, so writing new firmware should be doable. Now, to check what pin does what.