ESPHome

ESPHome compiles custom firmware for cheap ESP32 and ESP8266 microcontrollers from a YAML file: declare your sensors, relays and displays, flash once over USB, and update over WiFi forever after. Devices integrate natively with Home Assistant.

There is a learning curve around GPIO pins and the occasional soldering iron, but no cloud, no vendor app, and no device that stops working when a company shuts down its servers.

The gateway drug from buying smart devices to building them — a few dollars of hardware becomes exactly the sensor you wanted.