Q1 - Am I correct that the AirPort Extreme will essentially be acting as a network hub? Do I turn off wifi networking on the AirPort Extreme or set it to the same network name as the Mesh?
Not sure what you mean by "network hub". If you connect the AirPort Extreme(s) to the network using Ethernet and turn off the WiFi on the AirPort Extreme, the Extreme(s) will act like a simple Ethernet switch, to which Ethernet devices could be connected. There would be no reason to configure the AirPort Extreme to provide a wireless signal, since the Mesh network would be taking care of that function. If you did configure the AirPort Extreme to provide a WiFi network using the same name as the Mesh network, the AirPort Extreme would not perform as part of the Mesh WiFi network.
Q2 - I assume I need to reconfigure network settings on the Apple TV4K to the new Mesh network and it should work. Is this correct?
In theory yes if the Mesh system is backwards compatible to older WiFi standards. But, it would be better to connect the Apple TV to the network using a wired Ethernet cable connection if possible. Can't speak for Eero, but my neighbor was able to connect his Apple TV to a Netgear mesh WiFi system. He does have "freezes" on the video from time to time, so he is planning to connect the Apple TV using Ethernet in the near future.
Q2 - After configuring the MacBook Pro to the Mesh network, will I run into any issues trying to reconfigure backups?
What kind of backups? Will the backup hard drive be a USB type drive that connects directly to the Mac? Is it possible that your "AirPort Extreme" is really a "Time Capsule"......a WiFi router with built in hard drive designed to handle backups made using Time Machine? An AirPort Extreme is a router only......no hard drive on board that product.
In another room where I will have a Mesh satellite, I have another AirPort Extreme. Connected to that via Ethernet cable are a Mac Mini, and a Ethernet over power device that allows connection to a NetGear device in a back yard shed. If the AirPort Extremes are essentially acting as network hubs, then I should only have to reset them to connect to the Mesh network.
You will need to reset the AirPort Extreme(s) and then reconfigure them to connect to the Mesh network using an Ethernet connection. Once the AirPorts are set up, they will still be providing a WiFi network by default. You can then go in using AirPort Utility and turn off the WiFi function on the AirPort. In other words, Apple makes you set up the AirPort Extreme initially with WiFi. No way to get around that, so you have to later turn off the WiFi if you don't need the WiFi signal from the AirPort(s).
I have 3 AirPort Express units that I use for Airplay connection to old audio equipment. From what I have read, these can be reset to join the Mesh network wirelessly. Is this correct?
In theory yes, if the Mesh network is backwards compatible with older WiFi standards.
I have another Apple TV4K, and two 3rd generation Apple TV’s that would need to connect wirelessly. Will I run into any issues there?
In theory no, if the Mesh system is backwards compatible to older WiFi standards.
Keep in mind that Apple never offered WiFi 6 products, and even the most recent version of the AirPort Extreme was designed over 8 years ago. The most recent version of the AirPort Express was designed over 9 years ago. Mesh was largely unknown at the time, and Apple never made any hardware modifications or offered any specific updates that would allow AirPorts....or Apple TVs.....to connect to a mesh system.
