The real solution will be to more or less just add more (and better) Zigbee Router devices to your Zigbee network. So simply read and try to follow the tips in this → Zigbee networks: how to guide for avoiding interference and optimize for getting better range + coverage
You should also take actions regarding the advice there about EMF/EMI/RMI interference too.
You can not force a Zigbee device to connect to the Zigbee Coordinator or to a specific Zigbee Router device, load balancing is done totally dynamically. Again, all you can do is add more and better Zigbee Router devices.
It is done fully automatically and you have no contol over it other than adding more Zigbee Router devices and strategically placing them in locations. For more information checkout these:
https://community.home-assistant.io/t/zigbee-buyers-guide/654695
That is not true. While you can initially join a device via a specific Zigbee Router device in either ZHA or Zigbee2MQTT there is no way to force it to stay with a specific Zigbee Router, instead, all Zigbee devices constantly reevaluate its possible connections and should automatically connect to the best Zigbee Router device for it about once every 24-hours.