If streaming gives problem I guess it is not about bandwidth but the processing power of the device or perhaps faulty unit. Cheap unit tend to have slower processor thus hung up if buffer gets overloaded I think.

The difference between Deco Mesh and OneMesh has nothing to do with dedicated backhaul. When Asus came up with AiMesh, you just need to upgrade a bunch up listed models with AiMesh firmware and you can have mixed Asus model to perform like mesh network. Other players at that time you need to buy a mesh units set. If you already have one model, you have to ditch that one and replace with mesh set.

To simulate what Asus did, then TP came out with OneMesh. The Deco Mesh comes as a set of AP decicated for mesh. Therefore the OneMesh is cheaper solution. The difference is here:
https://community.tp-link.com/us/home/stories/detail/536

I do not have mesh routers but if I need to setup one, I will opt for LAN connection between mesh. The reason is simple. The connection speed between two unit related to distance and also obstructions. So, the further the distance, the backhaul speed will also suffer. You will have improved coverage but you will lose speed and not utilizing full router potential if you use wireless backhaul. You do not want to put your mesh unit near to each other right?

I bought an Asus since it was the only brand that enable you to build mesh network stage by stage since can mix model. However, my Asus GT-AC5300 can get at least 200Mbps anywhere inside my two storey house when I run speed test. Maybe not much thing in my house

My Unifi fiber goes under porch roof and through the wall behind TV cabinet and to BTU in TV cabinet. All hidden. I drill the hole and made preparation a day before the contractor came. So, it done very fast.

If I want to have a mesh router on 1st floor. I will route the cable the same way. Put short trunk outside the house and drill a hole in master bedroom where I can place a cabinet or shelf to put the unit. Will be nice without messy wiring inside the house.

Yes, single router is easier. Since now everyone gone AX crazy, the AC price much cheaper. Actually the old top tier router has some feature of the AX already. The modulation even go to QPSK 1024.

I'm using GT-AC5300 which is the same as RT-AC5300 (except for gaming feature I guess). My house is same two floor around 3000sqrt. My router is located in hall next to TV against wall facing car porch. I could still get around 200Mbps in closed room on upper floor leaning against the back wall of the house, tested using my cellphone @5GHz. My line is 500Mbps.

That silver box is the BTU (Broadband Termination Unit) which convert to and fro between optical and electrical signal. That device belong to service provider and under their responsibility if faulty whatever.

Consumer side starts from router.

I live in two storey semi-D house too. I bought Asus last time because during that time Asus was able to do mesh with different Asus router model, no need to buy one set of Mesh router at once, offered by other brand. That way I can add router later when needed.

I bought the Asus GT-AC5300 and place it in living hall. I tested the speed that my phone can established in far corner of a room with door closed and I still get 200Mbps. So, still using it until now and never need another router for mesh setup