@douwehoeben - For the OpenTherm capable boilers, I did find my perfect solution!
And it's here on the forums. Take a look at OT-Thing!
Tado cloud gone! Setup mine in master mode, I love it, only my tado TRVs stayed (fully local) and my external temp sensors in every room (cheap timmerflotte ikea temp sensors).
Then my automations and the tado x proxy integration - I built a very powerfull resilient winter setup 
I've just ordered a spare one today, just in case, since its being rather cheap at 50 euro (full tado x setup cost me 700 euro :).