Hello @davmoz
Re Modbus RTU: using Arduino code (not ESP-IDF) I can control an M5_ACSSR (relay) from an M5Tough just fine (using GPIO27 as RX and GPIO19 as TX). So I guess the GPIO assignment is correct, but I cannot tell whether your issue is with ESP-IDF or the SDM630, sorry.
Re Tough Tools firmware ext I2C: The ATECC608B is a 'bad' example as it requires special timing to wake it up and as long as it sleeps it doesn't show up in an I2C scan. Can you try with another I2C slave? I had success with an M5Stack PbHub for instance.
Thanks
Felix