here's the floorplan.
The red cross is where DB is and also the ONT.
the blue cross is the living room with another router
I have the tv connected via wire backhaul to the ONT
All rooms are lan connected but i do not use it at the moment. Currently, only use it for study room and living room.
Hope i got it right... thanks!!  
		 
		
	 
	
	
	
		
		
		
			
		
		
	
	
1. No so sure where is your study room. The above floor plan shows MBR, BR2, BR3, BR4, BR5 and Living Room.
2, WC uses ONR. Not so sure which ONR you get. No big difference though. 
ZTE F8648P ONR --> one 10G LAN port and four 1G LAN ports
Nokia XS-240X-A ONR --> one 10G LAN port and three 1G LAN port
3. HB710 can work with other ISPs. 
But I am not so sure if HB710 supports TP-Link Easymesh in AP mode or not. You may want to check with TP-Link Singapore. 
Reference: TP-Link EasyMesh in AP mode
https://community.tp-link.com/en/home/forum/topic/697572
The problem is that TP-Link HB710 (or EB810v or HB810 or HB410 or HB611-SGST) are TP-Link service provider models and the support is not as good as consumer models (less FW features, less frequent FW bug fixes). That being said, HB710 seems to be pretty stable and I like it myself (used as the main router for Starhub 5Gbps plan but I am not using a mesh).
3.1) Option 1 
If  HB710 supports EasyMesh in AP mode, then you can start with two HB710 first to see how it goes.
WC ONR 10G LAN port --> Living room LAN port --> main HB710 in AP mode
WC ONR 1G LAN port --> MBR LAN port or BR3 LAN port --> second HB710 in AP mode
You may need three HB710 if two does not work well.
WC ONR 10G LAN port --> Living room LAN port --> main HB710 in AP mode
WC ONR 1G LAN port --> MBR LAN port --> second HB710
WC ONR 1G LAN port --> BR4 LAN port --> third HB710
3.2) Option 2
If HB710 does not support EasyMesh in AP mode, then it is not a good option. Deco XE75 Pro is a fine option. Archer BE230 can be cost effective option (about S$85 BNIB in Carousell). Both Deco XE75 Pro (Deco Mesh) and Archer BE230 (TP-Link EasyMesh) support Mesh in AP mode.
WC ONR 10G LAN port --> Living room LAN port --> main Archer BE230 in AP mode
WC ONR 1G LAN port --> MBR LAN port --> Archer BE230 
WC ONR 1G LAN port --> BR4 LAN port --> Archer BE230