Half-related question: the chip on the board looks like an STM32 microcontroller, wouldn't a 6-pin 8-bit PIC10 or similar µC be sufficient and cheaper for the purpose? And possibly use less power, a PIC10F322 in sleep mode with the watchdog timer enabled is around 0.5 µA, while a small STM32 is more in the 100 µA range in the best case.
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