Well, the coordinator is probably not the problem, and the soil sensors are probably fine, too; from what I have heard Thirdreality devices are generally quite good.

That's what I heard, too, and my wife is very happy with being able to know exactly when to water the plants, which is nice.

I trust you are using a USB extension cable for the coordinator and don't have it plugged in directly in the USB port?

Of course. It's zip-tied to a plastic support on the shelving that holds everything. Well away from wifi, too.

No idea how good the smart plugs are, but if one of the soil sensors is basically next to the coordinator and still falls off the network randomly then the problem is likely not the plugs or the network mesh.

Ah that was worded weird - it's one of the smart plugs that's next to the coordinator. The soil sensors are all on other floors.

All things being equal I would suggest switching to zigbee2mqtt and see if that helps. Even if it doesn't, and the culprit is something else, zigbee2mqtt is (in my experience) better in the long run, because it generally supports more devices than ZHA and is much quicker in getting new devices supported.

That was my thought as well.