As @ninjatill points out, the wear leveling algorithm in the DeviceOS ensures the emulated EEPROM will last well beyond your product’s lifetime in your case. Nonetheless, @ninjatill does a great job of explaining your different options. Note that FRAM is available in both I2C and SPI versions and some folks have used a rechargeable CR2032 battery to power the retained RAM (and RTC) in the Photon during power loss. However, with proper hardware design and good exception handling in your code, the emulated EEPROM method is a good approach.
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