Zigbee is a mesh network so it’s a bit different. Zigbee devices can be an end device, or a router device (in addition to whatever it actually does likes controlling a plug). Routers contribute to your zigbee mesh, and devices connect through the mesh. This means you need more router devices to have a strong mesh. I use a few plugs where my network is weak, but otherwise have found that devices are stable. I can’t see on the zigbee2mqtt site if they’re a router or end device, but most powered devices are routers. ZHA and Zigbee2mqtt both tell you the device type if you go digging.

If they’re zigbee devices I don’t think they can be flashed with esphome. They’re not normal esp devices and that would likely disable the zigbee networking.