Personally, I would leverage the poweline ethernet to host a 2nd Hubitat. I would use that device to talk to all your garden devices, and use the Hubitat Mesh to link the hubs, and expose your devices. I use the mesh and I have power line ethernet too, but what I do not do in this example, is to run the hubitat through the power line ethernet, because I serve both hubitats at the same room. I can speak from experience that the power line ethernet devices are a true bridge device, so there is no filtering. As long as both power line ethernet devices are on the same leg of the the transformer, there should be no problems, and the devices will perform as if they were native to the main hubitat.
I am always weary of products like the ZBBridge, because Zigbee is not ethernet based, and cannot work over ethernet, except to be encapsulated into an ethernet frame, which then means that these are non-standard, and require specialized software to interact with the zigbee devices.
I read the manual. They call it a bridge, but it is not. For this to be a bridge, you would need two of them, and they would use ethernet as the mechanism to span the bridge allowing all zigbee devices on one side to see the zigbee devices on the other side. This is technically a gateway. It allows you to use one protocol (wifi) to communicate with device on another protocol (zigbee). The way this works, is that you install their app on your phone. Then using the app, you join the device to your wifi, then pair all the zigbee devices to their module. Once paired, you can control then using their supported wifi devices, which include Alexa. This type of gateway is the same thing as our smart home hubs. So why they call it a bridge is beyond me. IF there is a hubitat interface for the sonoff bridge, then it would be an inexpensive way to accessing those devices. Otherwise, to keep it simple, I recommend some Zigbee range extenders, or just using a 2nd hubitat.