So your issue is you made a poor choice at purchase and haven’t saved exactly what they said you could save up to?
Non of that is Tado’s fault. When Tado produced it’s financial model to substantiate it’s claim’s, they didn’t know you’d buy one, what house you’d live in, where that house would be, what it would be constructed from, how thermally efficient it would be, what type of fuel and how efficient your heating system would be, how your life/circumstances would be, what the fuel duty surcharges would be, your tariff pricing or what the weather would be like from one year to the next.
I paid about £60ish for my Tado, it’s saved me many times that over the years and that’s ignoring the convenience factor of going from no remote thermostat to having one. My parents got a Hive and use it to control a remote property at the other end of the country, it’s saved them driving thousands of miles or paying thousands to heat a house on oil when it wasn’t required. I can’t directly compare one year against another because one year we spent the majority of the summer away from home, another had a lovely long summer and another saw a significant change in family lifestyle. To try and directly compare them doesn’t work no matter how you try and fudge it.