Okay, so a bit of an update on my end... Lets start by attaching a diagram on my main floor (Floor 1).
Several important things to note:
1. Router position in the hallway is
fixed (hanging on the wall and cannot be changed).
2. My office is
exactly below Room 1.
3. All devices used on floor 2 get Wi-Fi from a Xiaomi Repeater (lets name it Repeater A) placed on that same second floor, just above the position of the AC66U router. I'm not drawing a diagram for floor 2, cause Wi-Fi is mainly used in the room where the "Repeater A" is situated in, or the room next to it, so signal is decent (reaching max speed and coverage is not that important on that floor).
As mentioned before, the connection up to this point was made as follow - PC in the office is connected to the Xiaomi Repeater in Room 1(naming it Repeater B) and is getting ~ 25Mbps from the 100Mbps possible. Every other position of the "repeater B" results in worse performance. Just for the record - connecting my PC to the router directly gives me an unstable connection with max speed of 5-10Mbps, so I'm not doing that. All other devices that I use on Floor 1 (e.g. phones, tablet, laptop) are connected to the router itself. The red circles indicate the zones I'm having unstable connection (almost no connection actually) and in general, the rooms where you see red circles in, don't have a decent Wi-Fi connection.
Yesterday I pulled the trigger and purchased the Deco M5 Mesh. Placed the main node in Room 1 near the router (there's a hole in the wall so it's easy to get a UTP cable from one device to another). The 2nd node I placed in the office below. That way my PC reaches 80-90Mbps which is insane compared to the connection till now. Haven't placed the 3rd node on floor 2, but I'm sure the result will be identical. My only problem is that if I connect all my other devices that I usually use on floor 1 to the mesh network instead of the router, the red circles become a lot bigger (almost no connection in the entire Room 5 for example). I can still leave them connected to the AC66U, so that's not that big of an issue, cause most of the devices don't move from one floor to the other, but it's annoying for my phone, cause I'll have to change the network every time I I swap floors (connection doesn't drop completely, but it's almost unusable).
One more thing I tested, was to place the main node right next to the router in the hallway. With the node in the office, I'm able to get ~ 35Mbps if I connect the PC to the mesh network, opposite to the 5-10Mbps I was getting if connected to the Router directly. The 35Mbps are almost identical to the speed I was getting with my previous connection to the Repeater B and another device on the wall looks ugly, so won't use it like that.
In summary - what I managed to achieve with the Deco M5 Mesh was to get the speed I was trying to reach in the office, which is basically the most important part. Floor 2 will also get better Wifi connection and coverage for sure with the 3rd node. What I didn't manage to achieve, was to have full coverage of floor 1. Also an issue is that I manually would have to change the Wifi network I'm connected to on my phone, which is extremely annoying.
And now some important question:
1. What would happen if I name the M5 Mesh network the same as the SSID of the AC66U I'm usually connected to on Floor 1? Will that mean that I'll be getting the best out of the coverage of the router when I'm in room 5 for example, but at the same time, I'll be getting the network speeds of the mesh network when I go to the office?
2. Are there any setting that I need to change on the Deco app in order to increase performance, or leave it to default? I updated the firmware already, but that's about all I did.
3. The cable connection inside my home is a bit tricky - The Fiber-Optic cable that delivers the internet goes inside a device which has a UTP out. A UTP cable connects that device to a 4 port Switch. 2 of these ports are for a wired connection to 2 x IPTV Set Top Boxes, 1 port is connected to the AC66U, the last port is free. I can either connect the main M5 node to the 4th Switch slot or the router. What is the difference, because either way I'm getting a MESH network running without issues?
4. I'm able to get an ASUS AC88U for a decent price (AC86U is not really an option, because there's no way to hang it on the wall out of the box). Will I be able to get a better Wi-Fi connection if I take a different approach - return the Deco M5 system, get the AC88U instead and place my existing AC66U in the office as an AP?
5. Either way, am I able to still use the Xiaomi Repeaters somehow, so they can help for these bad spots on floor 1?
Please keep in mind - I'm trying to achieve a better connection overall (mainly in the office), but desperately want to be able to use a single SSID and not having to swap Wi-Fi networks all the time.
Again, thanks a lot for all your help guys!