@JontyG 

What you have with M5 is expected: each hop cuts speed by half.

Still, Main Deco can do better: in my tests with gigabit Internet link Main Deco M5 can deliver up to 500Mbps.

Before replacing M5s, it would be good to check if Internet link is a bottleneck.

What is the speed of your Internet link?

Also, run "Test Internet Speed" from main screen of Deco app, that will report actual speed of your Internet link. What is it?

  @Alexandre. Thanks for your reply. 

I'm in Australia, where our Internet speeds are not great. 

My actual Internet speed is ~250 - 270Mbps, which is what I'm recording at main unit, and what Deco internet speed test reports

  @JontyG 

If you could sign for 500Mbps Internet, that would be the easiest solution. It should double WiFi speeds you see.

If not, 3-band Deco is an option, but do not expect miracles. Dual-band such as M5 cuts speed by half, tri-band in my tests cuts speed by 20%-25% at each hop.

With Internet speeds under 500Mbps you could use tri-band Deco not necessarily with 6GHz backhaul band, 5GHz can do, too. Here are options I'd like to recommend you:

M9 Plus

X90

X95

XE75 (the one you are considering)

Here is the thing: all can do, so you can just buy them based on price. You will need 3-unit set.

Can't help with router replacement, but XE75 is nothing special. If you can replace router with M5, you can with any other Deco. If you can't, I am not sure you can replace your router with XE75 or any other Deco model from this list.

  @JontyG I'm not sure about the HFC connection. Umm ... don't you have coaxial cable going into the modem?

If you do then I think the modem serves as a media converter from the coaxial cable to the local network, the xe75 has no modem it just has ethernet and hence can connect directly to the ethenet port of the fibre nbn box (which I don't think you have).

I wonder if you need connection information?

Does your modem(/router) provide phone services?

What provider are you with?

  @raven-au thanks for your reply. 

I have coax I to NBN box, which connects via ethernet to my AR1600V modem/router. 

I'm with iinet. Don't have phone services configured, but do use dynamic DNS and port forwarding. 

JontyG wrote

Unfortunately a wired backhaul is not an option for me.

Thanks for the responses. 

Having looked more closely at my setup and options I've decided I'll install an ethernet backhaul through a switch and stick with the M5s for now.

I was hoping to avoid this (I hate running cable), but understand this will give me the best performance outcome and bang for buck. Just have to find some time and motivation to do it 😉. 

  @JontyG if you can use wired backhaul that would get you the best improvement, particularly as you have dual band devices.

The reality is, in many cases, you don't get as much improvemnt as you had hoped with wireless backhaul.