If performance and stability is essential your best bet is to fit a couple of Ubiqiuti UniFi AP's wired back to the BT HH5. Disable the WiFi on the HH5 and just use the UniFi kit. You can get UniFi AP AC LR's for around £100 each.
I have never connected the BT Whole Home Wifi to a BT Hub so I am not aware of any issues. I always found the hubs to be unreliable and thus defeats the point of the BT Whole Home Wifi.
However, thats not to say you can't. You would need to ensure that the BT hub is acting as your DHCP server on your network as the discs initially require a IP address to get you started.
The first disc should be hardwired directly back to your hub and switched on. The rest is done though the hub.
Set up the wired disc first using the BT app before turning on any of the other discs.
What is the problem you are experiencing then?