This, honestly, is the best advice. Thank you, @christatt, for suggesting it. On the back of this random stranger’s advice on the Internet, I got myself a few more second-hand TRVs and the Wiser Hub. Most of the house is now zonally controlled and it works better than I could either have hoped or created myself. The HA integration too is just superb.
Why did I not do this in the first place? Probably because I didn’t think buying hardware from the same provider would or should provide any benefits. Anything the Wiser software can do would surely be replicable within Home Assistant, right? Well, no. There’s some clever energy-saving intelligence which seems to do well at using the minimum amount of energy for just the right amount of heat.
It’s not 100% ideal. I’d just finished investing in a solid Zigbee network based on Z2A, which is incompatible with the Wiser kit. I now have two Zigbee networks, each with their own extenders, and that feels like a waste. The Hub itself requires WiFi, which is patchy in that area of the house and seemingly prone to radio noise from the boiler itself. And at the back of my mind is a suspicion that, if Drayton / Schneider had really wanted to, they could have made the Hub and TRVs properly Zigbee compliant instead of extending the standard for their own requirements.
But these are minor niggles. Do yourself a favour: listen to @christatt.