I have never read any mention of BT/BLE on the Rachio controller, besides, you have a radio issue with anything almost buried into the grass. Bluetooth is probably not the right technology as it will not allow for far enough placement. Something z-wave or around the 433-866MHz spectrum. Another problem here is the divergence between US and Europe/others on available spectrum at sub-gigahertz frequencies (lower frequencies go farther) meaning sensors and controllers may need to be country specific, a logistics problem. BT 5.0 promises longer range, but still uses the 2.4GHz, also shared by wifi and Zigbee.
I do want the likes of Moen sensors though, the ability of multiple depth measurement seems awesome. Have you been able to use any, without their controller?
I have Rachio and the Ecowitt sensors connected to Hubitat, so I can technically turn on a zone if it becomes too dry, but I really don’t want to bypass the Rachio logic, a sensor baked into Rachio directly would be phenomenal 
LE: according to Ecowitt, their WH51 sensor uses 915MHz in the US, 868MHz in Europe, and 433MHz elsewhere.
I cannot find any info on the Moen one, searching FCC…