I bought some ESP32S modules to use to collect the data from the sensors.
There are ESP32S libraries available to read the Xiaomi (and compatible) sensors, and the ESP32S has Bluetooth and WiFi to collect the data and transfer it to a central data logger.
I also bought a bunch of solar charging power banks. I'm going to modify them to produce a continuous 3.3V for the ESP32S modules.
That will involve removing the cigarette lighter heating coil from the power banks, and installing an LM3671 regulator module.
The regulators will be connected to the internal lithium polymer cell via the existing undervoltage protection circuit.
I'm waiting on delivery of some PTC fuses to protect the 3.3V output before I begin modifications.
I am also going to include a battery voltage monitor input so that my system can warn me if the solar charger isn't adequate.
The current plan is to park the power banks on a fence post in the yard with the ESP32S plugged into the power banks.
Rather than try to waterproof things, I'm going to put large plastic soda bottles over the top of the posts to keep the electronics out of the rain but still allow enough sunlight to hit the solar cells.
Current state of the collector units: