I would suggest the following options :
Wired back-haul :
1) Replacing the XT8 inside the DB box with a wired 4-6 lan ports router like Microtik router or UBNT Edgerouter X or even your own pFsense router. For the Living Room XT8 and Bdrm 2 XT8 can connect to the router forming a AiMesh environment.
ONT <-----> Wired router
........................|
........................|(Router Lan port 1) <-----> Living Room XT8 Main node - AP mode (Wan port)
........................|
........................|(Router Lan port 2) <-----> Bdrm 2 XT8 (Wan port)
Pro : You save costs vs option 2 and potentially gain more router feature such as firewall, QOS control.
Cons : U will lose some feature of Asus like DDNS and
remote Asus App management of your wireless XT8 setup. And you need to learn how to tinkle with those wired router setup.
2) Go with your original planned setup but minus the switch since XT8 has 3 lan ports behind
ONT <-----> XT8 Main router
........................|
........................|(Router Lan port 1) <-----> Living Room XT8 node (Wan port)
........................|
........................|(Router Lan port 2) <-----> Bdrm 2 XT8 node (Wan port)
Pro : You have a full XT8 setup and able to control management all your XT8 units
locally and remotely using web and Asus mobile apps
Cons : U will need to spend a bit more money vs option A. And you may have overlapping wifi zones *eg DB + Living room* and potentially wasting the tri-band bandwidth of XT8.
Wireless back-haul
3) Connect ONT to the living room using the LAN pt and move the Living Room XT8 to in between DB box and the lan point and facing the corridor towards the room. Connect a 2nd X8 via
Wireless AiMesh in either your Bedroom 2 or MBdrm.
ONT <-----> (DB Living RM Lan) <-----> Living Room XT8 Main Router node (Wan port)
........................................................................|
........................................................................|(XT8 dedicated 5GHz band) <-----> Bdrm 2/MBedroom XT8 node
Pro : Cheapest of the 3 options. And you have a full XT8 setup and able to control management all your XT8 units
locally and remotely using web and mobile apps. Of course this is the least power consumption and you dont have to worry about the ventilation / heat issues inside the box
Cons : Wifi speed in Bedroom 1 2 and MBdroom is def slowly than wired back-haul but def more than enough for most usage.
Not sure what will be your usage patterns in those bdroom. I did the same for my house. You can read it over
here. I am able to get 300-400Mbps consistent and stable speed using the wireless back-haul approach.
PS : In fact there's alternation option - a variant of 1) . that's the replace the
XT8 in option 1) with
XD4 assuming you wont need any LAN connectivity in Living Room or Bedrooms. This option is potentially the cheapest option among all.