Building a huge house that is 1500sqm with probably have more than 400 Zigbee devices would it be better to buy 2 Zigbee routers and run half of the 400 devices (200) on it’s own Zigbee2MQTT and the other 200 devices on ZHA? Would this be better so as not to have maximum capacity problems?
Second question are there any smart home professionals I can hire to design my whole system for device compatibility, automation and dashboard design?
Probably, either that or two or more instances or Zigbee2MQTT with each its own Zigbee Coordinator.
At least if you are talking about Zigbee Coordinator with its radio in NCP mode and Zigbee 3.0 devices, Zigbee 3.0 devices have larger security overhead which means that current Zigbee Coordiantor adapters can only handle about 200 Zigbee 3.0 devices in total, while they can handle several thousands of devices if you are talking about older Zigbee Home Automation or Zigbee Light Link devices.
CC2652P based Zigbee Coordinator (used for Zigbee2MQTT) can max handle 200 Zigbee 3.0 devices when in NCP mode. https://github.com/Koenkk/Z-Stack-firmware/blob/a7745634b539df3d0b1ad1ae84a137017af58fe2/coordinator/
EFR32MG21 based Zigbee Coordinator (used for ZHA) can handle around 200 Zigbee 3.0 devices when in NCP mode (probably a little more devices than CC2652P as EFR32MG21 has more RAM).
An upcoming EFR32MG24 based Zigbee Coordinator adapter (used for ZHA) has a lot more RAM on the MCU and can therefore probably handle more than Zigbee 3.0 devices when in NCP mode.
If the radio is in RCP mode dedicated for only Zigbee Coordinaor (and used as a multiprotocol Multi-PAN RCP radio) instead then it is possible that it could probably handle more Zigbee 3.0 devices since it then offloads part of the Zigbee stack to the host computer instead of running the full Zigbee stack locall on the radio MCU as it does on NCP mode. See https://github.com/home-assistant/addons/blob/3af0ad8a398dd6d969e98ca8a5c5e2ead754f365/silabs-multiprotocol/DOCS.md#architecture
Regardless read and follow this → https://community.home-assistant.io/t/guide-for-zigbee-interference-avoidance-and-network-range-coverage-optimization/515752