[willglynn]

ESPHome fills much of this niche for me. It's a framework for turning YAML device definitions into custom microcontroller firmware, with myriad supporting tools. The official device database at https://devices.esphome.io lists 554 devices but that's nowhere near the end of it.

Most manufacturers bolt on IOT functions by dropping an off-the-shelf module onto their device-specific board. It's sometimes possible to replace the factory firmware with ESPHome, sometimes even using over-the-air updates. For example, AirGradient air quality sensors: https://github.com/MallocArray/airgradient_esphome

Even when it isn't possible to commandeer the factory IOT module, the fact that it _is_ a module is still useful, because it's almost always possible to inhibit or remove the factory module and connect your own instead. The factory IOT module controls and senses the device, so your replacement module can too, using the same pins. For example, an IOT air filter: https://github.com/mill1000/esphome-winix-c545#final-assembl...

Some devices are designed around multidrop communication busses. These are usually even easier, since the ability to join the bus is an intended design feature, even if the device you're using is not intended. For example, many Samsung residential HVAC systems: https://github.com/omerfaruk-aran/esphome_samsung_hvac_bus/d...