@NugentS to clarify, have you bought ITead’s “Sonoff Zigbee 3.0 USB Dongle Plus” (model “ZBDongle-P”) or ITead’s “Sonoff Zigbee 3.0 USB Dongle Plus V2” (model “ZBDongle-E”) and will you be migrating your devices to either the Zigbee Home Automation (ZHA) integration or Zigbee2MQTT now or?
Regardless, I posted an extensive reply to your questions in this other relevant thread instead or here as at least my answers and suggestions can help more people there if they are wondering the same things → Zigbee networks: how to guide for avoiding interference and optimize for getting better range + coverage
Short answer is that you should add many Zigbee Router devices, and “known good” ones work better.
To get the best of both worlds it is best to add both a few dedicated Zigbee Router devices as well as loads of non-dedicated Zigbee products (mains-powered and act as Zigbee Routers). Zigbee uses mesh networking and a Zigbee network will work better and better with the more Zigbee Router devices that you add to it.
FYI, dedicated Zigbee Router devices are known to have much better reception and transmission + can often route more devices than non-dedicated Zigbee products such as smart outlet power-plug,
For example, it is well known that IKEA Tradfri Signal Repeater works much better as a Zigbee Router device (better reception/transmission and can handle roiting for more devices) than their IKEA Tradfri Wireless Control Outlet power-plug.
There is much more discussion on that and related topics here → Zigbee networks: how to guide for avoiding interference and optimize for getting better range + coverage