This is a great project, one that I would like to replicate at some point. But I probably have counterfeit DS18B20 probes! (Thanks for that link.)
I'm a little concerned that you don't appear to have any failsafe mechanism. When I've fiddled with DIY thermostats, I've always had a mechanical thermostat set at 40-45 F in-parallel with the experimental thermostat. That way the heat will still kick on and keep the pipes from freezing if the project goes kaput and I'm not home.
For your project, I'd recommend a mechanical thermostat (low temperature range) in the coldest room of the house, set at 40-45 F, and wired to activate at least one heating loop valve and the pump. I would also add an external watchdog timer to reboot the Pi if it stops sending a heartbeat every few minutes.
For those who disagree with the article's premise, there is Devuan.
https://www.devuan.org/
It's a mature fork of Debian that preserves init system choice.