You don’t need a smart thermostat: that is what HA is for.
You need one or more temperature sensors, and the ability to turn your heating on and off from HA.
Then you build all of the smart functions into your HA configuration.
There are lots of simple temperature sensors available. I have built my own using an ESP8266 running ESPHome and using a simple DS18B20 sensor in a 3D-printed case, powered over the wiring that used to connect my dumb thermostat to the heating unit. Although it looks fancy with its OLED screen, it is simply working as a temperature sensor.
I’ve also replaced the heating unit with another DIY unit using an ESP8266 and a couple of relays controlling the boiler and the hot water controls.
My HA setup then uses the Generic Thermostat and a load of logic about when the heating should run — fully controllable from anywhere with the HA app, and automatically switches off when everyone is out:
This has been running completely reliably for more than three years now. Fully local control, doesn’t rely on any cloud connection.
So your first step is to get HA up and running (plenty of hardware options: I run mine in a Docker container on my NAS) then get the ability to control your heating from HA. Everything else follows from that.