Ideally there'd be something both wireless and battery powered that could do it all (like the Tempe but with Humidity and Pressure) but I don't want to be changing batteries in things all the time - and non-rechargeable batteries aren't great for the environment.
I did wonder about using a bunch of Pi Zero powered using PoE but since there's no ethernet wiring in the walls of the flat that's not a goer. If I'm running cables then they may as well be a big passive 1-wire loop.
Even those IoT4.eu sensors need power and have a cable running to the thermocouple so there's some form of wiring already. The main reason for not using them is that I generally want two temperature sensors in each room, one for air temp and one for radiator pipe temp. Paying another EUR29 just for an extra temp sensor is a bit much.
If I was able to splice in a second DS18B20 of my own into the wire and the system was capable of dealing with this then the price (including the humidity sensor) would actually be quite reasonable.
The electronics and backend coding bits don't bother me, one of the previous projects was a RPi that would scrape the TFL live bus website and display the time and when the next local bus was due outside our house (it only goes once every half an hour). The Pi ZeroSeg module needed to be soldered together, which is less fun with lead-free
sodder solder. One of the buttons would make it cycle through the buses due at the end of the road (Putney Hill), the other button would make it display a silly message for my daughter.
(I think I used a PiZero for that hence the separate Wifi dongle. Using a PiZeroW would mean I can get rid of that bit.)
I was going to place it in the hall by the front door where the existing alarm keypad is (and I can use the existing alarm cables to provide the 5V USB power for the Pi).