If you’re running HAOS, I’d start with the z2m add-on. No need for a separate CPU. It’s simple to move to a dedicated unit later if desired.
Then I’d probably go with a simpler USB stick.
Not really, only one coordinator per network. There are some situations like outbuildings where it makes sense to have more than one net, but most residential uses would be better served with a single well built mesh.
See @hedda’s zigbee optimization thread: Zigbee network optimization: a how-to guide for avoiding radio frequency interference + adding Zigbee Router devices (repeaters/extenders) to get a stable Zigbee network mesh with best possible range and coverage by fully utilizing Zigbee mesh networking