Hi a bit late to the party but this is my experience updating several both powers and battery-powered devices.
Powers devices will handle updates by themselves, as you stated. And are quiet quick.
Battery devices will start quickly if you select firmware and press update several times while using the device for example pressing the button on a remote.
But you can only update one battery device at one time.
If the device have a fresh cr2032 battery 100% charge. it will lose roughly 30-40% power during a update.
On battery devices it goes a bit faster if you move it closer to the conbee device. But still slow. Fastest one I had was 2h but brand new installs next to Conner stick but the average is about 6h. There will be slowdowns in the network caused by all packets being transmitted and if that goes via other routers that will make other things feel slow.
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