I ran into this exact quandry when my home thermostat died late last year. Didn't want Nest/Google nor anyone else getting my data. Furthermore, I'm rural so broadband is expensive and sometimes flakey, so that was another reason to want a local-net-only solution, but there were no good ones I could find.
So I just built my own smart thermostat based on a Raspberry Pi: https://github.com/chaeron/thermostat
Since then, I've added battery-powered remote sensors (temp, humidity, pressure, water detection, etc.) to monitor key areas in and outside of the house. These were based on Arduinos (to keep power usage low) and the open source MySensors framework (https://www.mysensors.org/)
Fun project, works great, no security issues or cloud crap.
More recently I've built on that experience and am working on a dirt bike telemetry project which is based on an Arduino Mega with a GPS module, 9-axis motion sensor, hall effect sensors (for wheel spin/RPM detection) and more. The device captures real time info about the location and movement of the bike (eg. pitch, lean angle, etc.) and then can be used to overlay gauges and a moving 3D bike model over top of headcam footage captured during a ride.