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Good question on the 4k, currently none of my TVs are 4k so not that worried by it yet.
Will have the master box driving the lounge
1 x mini box will drive my bedroom
1 x mini box will get split into 4 signals and sent to the three other bedrooms and the kitchen, so they will all be forced to watch the same channel, but even if I got another couple of mini boxes you can only use 2 at once so the others would be left with nothing, so this feels like the best compromise.
Similar to my setup, although I only use one Chromecast Audio hooked up to my separates system. To be honest I don't find the Chromecast very reliable, even via an Ethernet cable. The initial connection from whichever app you are using is pretty flaky, particularly with Spotify.
Regarding the Sky setup, are you going to use mini boxes for all your TVs? I don't think they do 4K do they?