+1 on all those points jerrm made.
While technically possible a Zigbee home automation-based solution would not be recommended as it is the use case for Zigbee and those devices mentioned are really meant for a single household/home.
Also, understand that Zigbee radios are very weak with poor material signal penetration, so each separate Zigbee network instance would need loads of mains-powered “Zigbee Router” devices as always-on Zigbee repeaters, (as battery powered devices are “Zigbee End Devices” that do not route/repeat messages). Read → https://community.home-assistant.io/t/guide-for-zigbee-interference-avoidance-and-network-range-coverage-optimization/515752
Do not know in which country you live but perhaps check out professional climate solutions designed for residential heating use from example https://www.uponor.com and/or https://www.danfoss.com
A compromise semi-professional could maybe be to instead get a separate system setup/installation of from https://www.tado.com for each apartment and then integrate all of those into Home Assistant.
Tip to look for if want a reliable solution is to go with something that at least is wired (without batteries).