@Alexandre. 

Thank you for answering my questions, also promoting a new Deco to main later seems to be a great idea.

As for your question, I want to upgrade to a tri-band solution, where the 3rd band can be used as a high bandwidth wireless backhaul (X60 could not as it is dual band), and most of the bandwidth is used by 2 units (main and central).

  @Amano7 

If you want tri-band, you can also choose from Deco X68 and X90. They are tri-band and based on same WiFi6 technology as X60, so if you are OK with X60 performance - Deco X68 and especially top of the line Deco X90 will definitely do better in your Deco mesh.

XE75 with WiF6E is a different iteration of technology. WiFi6E should be faster than WiFi6, but on shorter distances and it is more sensitive to obstacles. Which means, depending on your house layout and X60s placement, replacing them with XE75 may or may not improve WiFi speeds. You would not know, until you try.

XE75 has gigabit Ethernet ports. It means, your Internet connectivity and everything wired to Deco will be limited by gigabit, even if XE75 backhaul can deliver more. What is the speed of your Internet connection now and do you plan to buy faster Internet package from ISP in the nearest future?

If you haven't placed an order for XE75s yet, I can give you a simple way, by using X60s you have, to estimate if replacing them with XE75 will improve backhaul speed. That would not be an estimate with absolute certainty, but at least it'll give you a general idea. If in doubt, the better choice could be X90 or X68.

If you did order XE75s already - when they come and you replaced your two X60s with them, please share results.

  @Alexandre. 

I'm currently on 1gbps ISP (tested ~900mbps up/down from main unit).

Current X60 wifi performance is only 200-300 mbps at best.

I'm trying to avoid 5Ghz as the 3rd band, due my airspace is rather congested.

The central unit is wall mounted and has a line of sight to the main unit, plus wired is not an option due to a lack of ethernet cable infrastructure in my concrete home.

Also channels 50-144 in my country is DFS/TPC which requires to limit power so the range will be affected, plus disconnection when the nearby radar station decide to use any of these range.

Getting 6Ghz bandwidth will be better for future proofing (airspace congestion-wise).

  @Amano7 

That makes sense.

If you are concerned with future proofing, and plan to upgrade Internet connection to more than gigabit, check prices for Deco models with 2.5Gbps and 10Gbps Ethernet ports. Deco XE75 Pro and Deco X90 have one 2.5Gbps port and rest are gigabit. Deco XE2000 will have 10Gbps port, but I don't think it is released yet.

Alternatively, you can just go with your choice, XE75, and swap Main Deco XE75 with XE75 Pro or XE200 that you'll buy later, after you have your Internet upgraded and know what Ethernet ports your new ISP modem/router comes with (not every 10Gbps Ethernet port supports 2.5Gbps).