FYI, all devices are not equal and it is by now well known within the community that dedicated Zigbee Router devices do a much better job at acting as a Zigbee signal repeater / range extender than any standard Zigbee outlet power-plug or lightbulbs does at routing.
Dedicated Zigbee Router devices also have optimized firmware that allows you to connect more devices to them compared to a non-dedicated Zigbee Router device which will have a lower limit set in their firmware (with most non-dedicated Zigbee Router device firmware only allowing 5-20 devices to connect via them while many dedicated Zigbee Router device firmware allow 20-50 devices to connect via them)
Also, some bad devices do not support Zigbee routing for other brands → https://www.home-assistant.io/integrations/zha#best-practices-to-avoid-pairingconnection-difficulties
Yes a Zigbee network will become stronger the more Zigbee Router devices that you add but until you built out a large network it can be a very good idea to buy a few dedicated Zigbee Router devices → Zigbee networks: how to guide for avoiding interference and optimize for getting better range + coverage
As an example; IKEA’s Trådfri Smart Outlet Power-Plug is known to be much worse than their IKEA Trådfri Signal Repeater, both in signal reception and the amount of devices that it can support routing for.