Alright, your image is pretty clear. From my gut feeling, that should work fine and you should not encounter any major issues. These distances are within the typical operation range of both Zigbee and ZWave. But RF signals can sometimes be unpredictable and very dependent on the local area and topography. Number 4 might be a little problematic and might require a repeater.
That said, in the absence of any repeater, the signal is not going to travel from one sensor to the next. Each sensor will talk to the controller stick or hub directly. So you have to look at the distances between every Zigbee or zwave device (2, 3, 4) and the hub (1). And you have to think in three dimensions here.
Do this: go to each location where you intend to put a wireless device. Now imagine a virtual line going from the device to your hub or stick, like if you had a laser pointer shining into this direction. You can imagine the hub or stick being at varying heights. Is there anything in the way ? Walls, maybe parked cars, a tree. If you’re on the northern hemisphere and it’s winter right now, keep in mind that what can look like a clear line of sight now, can become much more problematic in the summer when vegetation gets dense. Leaves will absorb signal, especially in the GHz range. So try to find a good position for the stick or hub with the least distance and the least amount of stuff in the way of the signal from the sensors.
Sure, that can be a good option if you intend to use both systems. Some people report those sticks to work less well than ones dedicated to a specific protocol though. Something to keep in mind.
Understood. I don’t know enough about Wifi 6 to comment on it.
Typically any Zigbee or ZWave device that is connected to mains power will act as a repeater. That is automatic and does not have to be configured. But as always, there is the odd exception. So if you have a specific device in mind, you should probably do some quick research (or ask) if it can act as a repeater or not.
Unfortunately yes. Different devices will have different transmit power, different antennas, etc. Zigbee is more problematic than ZWave. The latter is a controlled standard with a tight certification process (that’s why it’s more expensive), so devices will have well defined technical specifications that you can look up and compare (look for tx power and receiver sensitivity). Zigbee is pretty much free for all, and you will find all kinds of implementations for it. Ranging from excellent quality to complete crap bordering on unusable. Usually you get what you pay for. Don’t expect some cheapo stuff from AliExpress to use high quality components.
No, it’s not dependent on the wifi. I assume you have the Pi connected to your network by wired Ethernet ? If you go with Zigbee, it could make sense to keep some distance between the Zigbee stick and your router, let’s say a meter at least, to reduce problems with interference. Also choose wifi and Zigbee channels so that they don’t overlap.