I will try to compare Hive with Nest, until this year Nest was the best, but then Hive brought out their own TRV heads, and what these do is give Hive a command to heat for 1/2 hour at a time if under the set temperature, so the wall thermostat is like a resettable timer which keeps boiler running if any TRV head needs heat,
Nest works opposite way, the wall thermostat tells the TRV head what temperature has been set, so they all follow the wall thermostat.
Hive does not have volt free contacts and is not open therm enabled, where Nest both DHW and CH are independent relays and has open therm.
I have Nest but would not say either are really good systems, from what I read it would seem EvoHome and Tado are better where the TRV head tells the wall thermostat what is required and the wall thermostat in turns tells the boiler what heat output.
However cost is also a consideration, a TRV head which can link in some way to the wall thermostat costs around £40 to £60 and that of course can add up, a stand alone head starts at around £10 I paid £15 each for bluetooth units as where they are hard to read and set.
I suppose with TRV heads there is no need for zone valves, I have two different programmable TRV heads one goes down to 5°C and other 12°C which is nearly the same as off, as rarely will any part of house get that cold. However like you I have an area of the house only used by guests so I have a zone valve to turn it off, and the zone valve includes a micro switch, so the thermostat controls zone valve and zone valve switches on boiler, but my boiler is oil, so does not modulate, it using a modern gas boiler the zone valve does not work in the same way, the programmer can work zone valve but thermostat needs to turn boiler up/down not on/off, so often a heat store is used.
With a heat store two boilers could work in tandem, as to if that is good or bad can't say, but the main problem with a modern boiler, is turning it down low enough so it may work well, however getting a bit ahead here as don't know if you have a modern boiler or not.
So with some thing like EvoHome whole idea is each room heated independent from one boiler, and to be fair with a TRV head in every room EvoHome compares well with Hive and Nest, where Hive and Nest gain is when you fit cheaper TRV heads in some rooms, although not sure with any system how many TRV heads need to be linked to the wall thermostat for it to work well? I have 4 TRV heads connected to Nest, the upper rooms still have programmable TRV heads but not linked to the wall thermostat, so can't have bedroom warm without some heat to entrance level floor, but will work other way around.
When I came to fit mine, I spend a long time deciding how to wire, and Main reason for Nest was 1) I already had 4 TRV heads that would work with Nest. 2) It only needed two wires to work both CH and DHW and that was all I had, so for me clear it was best, but for you not so cut and dried.
One thing however is how one can expand and start small. Nest e would likely do your ground floor easy, with Nest generation 3 for the system boiler then you can add Energenie MiHome TRV heads as you feel they are required. The same applies to Hive I think, the TRV heads can be paired to the wall thermostat, but will also work as stand alone. I in fact started with the TRV heads, in the last house I had a control problem with morning sun causing main living room temperature to sore, so fitted first 2 heads, which worked well so expanded to 4 heads, in that house the wall thermostat was never linked to the TRV's. OK had to set times to match, the programmer turned on at same time as the TRV heads changed their time, the problem room instead of jumping to 28°C when sun came out only went to 24°C and other than the sun in bay windows, the rooms were as set on the heads.
Not sure is EvoHome or Tado heads can be used stand alone, but Energenie and Hive heads can, and no wiring involved so easy to fit, if you don't want them linked the eQ-3 I fitted were really simple, and I have set Eco and Standard temperatures so one push of a button and it swaps from a low stand by temperature to room in use temperature, clearly boiler has to be running for temperature to change, but £15 is a lot cheaper than £40 even cheaper if no bluetooth.
