ESP8266 is cheaper than RISC-V and powerful enough. It's not open source but that doesn't seem to be a deal breaker for this project. 802.11ah has only 1km ran

ESP8266 is cheaper than RISC-V and powerful enough. It's not open source but that doesn't seem to be a deal breaker for this project. 802.11ah has only 1km range which is not enough for this purpose. LoRa is rated at more than 10km range at 292bit/s up to 50kbit/s. Also the chip used in this project costs only around $6.