If we ever sell this (which we don't plan to), I know what to install (it'll be quite good, just not this perfect). I have it in a cupboard (a Siemens Connected Home thermostat system), the downside of that is that the combined UFH+radiator rooms will be less comfortable.

(But still more comfortable than 99% of the houses I've been in.)

I haven't mentioned in the parent comment but as a test I've dismantled the HA system and installed the Siemens system and it works well, just not 'perfectly'.

Sounds well thought out. Many seem to forget designing for replacing.

In generall I think all these IoT systems will be a major headache as they age.

My thermostats on the radiators are 45 year old by now. That is kinda the expected service life we are used to.

The existing professional setup was also a nightmare, what gives?

Well first of I am envious and I would want to do something similar.

If I inherit a heating system I want it to be all mechanical except maybe the control system for any heating pump.