homeassistant

I'm starting to replace some wifi devices with zigbee too, Yes many zigbee devices will act as a repeater or router for other devices to connect through, the only devices that tend not to have this functionality are battery powered devices like pushbuttons or remotes.

Just a word of warning, those S26 Zigbee outlets have quite poor range so you might still run into issues if you only have a couple of them.

As far as I know there is no alternative firmware for the Zigbee and Z-wave devices. nor is there any real need for one as they do tend to "just work" but that also means you are stuck with the features and functions they give you. There are some Zigbee bridges that can run ESPhome to add some customisability, but that is just for the wifi side of the device.

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.

Zigbee devices are already cloudless local devices, you don't have to flash them unless there are alternative firmwares that add functionality you desire.

They also need a coordinator to comunicate with HA. You can get a zigbee stick, depending on how you plan on integrating these devices there are some chipsets you should look for. If you want the easy route go for ZHA and maybe the skyconnect stick, otherwise you can go with zigbee2mqtt route which supports more devices and is more customizable and look at their list of recommended coordinators.

Most mains powered zigbee devices act as router and extend the newtork, not all of them do and not all that do are good at it. Some manufacturers use custom implementations of zigbee like Aqara which can cause issues.

I use zigbee2mqtt with a sonoff P stick and I am happy with it, it's not as plug&play as ZHA, but it's easy to add support to new devices if they are not already supported.