The cheap smart thermostat needed a C-wire my 1990s wiring did not have. Instead of fishing a new wire, I added an adapter at the furnace and reflashed the unit with ESPHome for fully local control.
bill_of_materials
Thermostat
Generic ESP32 unit
Adapter
Add-a-wire / C-wire kit
Firmware
ESPHome
Integration
Home Assistant
verdictLocal control, no cloud, survived a winter. ESPHome means it answers to Home Assistant instead of a subscription.