That’s good that you have run network cable everywhere. THis should make things work nice and smooth.
For you multi room audio I would suggest something like:
8 x Raspberry Pi3 (as audio player for each room)
1 x Raspbarry Pi4 (as server and audio player for living room)
9 x DAC hats – something like HiFiBerry DAC+ (one each Pi)
This assumes you provide other amplification for each room. You could alternatively substitute the DAC hats with hat amps instead – something like: IQaudIO Pi-DigiAMP+ instead of the HiFiBerry DAC.
You would configure the Pi4 as the server by installing Logitech Media server from Max2play interface. To this Pi4 you also configure as an audio player and also install Kodi for your video needs (also from the Max2play interface).
The other Pis for each room you configure as players (Squeezeplayer) by using the Squeezelite option in Max2play.
To control it all you install an app for your smart phone. If Android then you can use Orange Squeeze and if Apple-based you can use iPeng.
If you want displays too, you can choose an option from the store. For example there is a 7 inch screen with holder for Rasperry pi on the back. You may find an elegant means to mount it or otherwise could purchase a case to house the display and Pi.
Using Pi 4 all round may be a better solution due to gigabit connections, but either should be fine.
Hope that helps as a starter.