@Benedictm 

Usually if everything works, people don't tend to redo their home network. Do you have reason to try separate router? Because, otherwise, the answer would be "yes, you can, but nobody can tell if you should."

  @Alexandre. that's a good point :) - i feel is sluggish and would like to speed up

i plan to get a faster fibre and start putting in wifi 6 decon instead

if I replace 1 0r 2 ac 1200 decon with wifi 6 deco will see a difference or dothey allneed to be swapped?

  @Benedictm 

Wired Deco M5 can't deliver speeds faster than 500Mbps, and wirelessly connected Satellite M5 can't do better than 250Mbps. This is what I observed in my tests.

If you want speeds closer to gigabit, you should get WiFi6 capable Deco. If your budget allows it, I would recommend to swap all Deco units. Otherwise, you will depend on devices capability to roam between WiFi 6 and WIFi 5 Deco nodes, seamlessly. If they can, good for you, but if some smartphone or tablet or whatelse can't - that's not Deco problem.

In mixed environment, mobile device capable of WiFi 6 will still get WiFi 5 speeds when it is connected to Deco M5. When it is connected to WiFi6 capable Deco, you'll see WiFi 6 speeds. Expect 800-900Mbps from WiFi 6 Deco that is wired, less from wireless Satellite Deco nodes.

If you are OK with mixing M5s with WiFi 6 Deco, first step is to replace your wired decos with WiFi6 capable units. This is where you'll see most improvement. Keep M5 as wirelessly connected Satellite Decos. 

Also, it does not matter much if you will add a third party router to your home network or make WiFi6 capable Deco your Main Deco in Router mode. As long as you don't utilize content filtering and bandwidth throttling in Deco mesh, its performance should be very close to what you could get from having a router and Deco running in AP mode.

My preference is to run Deco in Access Point mode, but if you need to buy a router for that, why spend money for little or no gain? If, on the other hand, you can have router from your ISP, for free or same monthly price you pay for ISP modem now, then get it and have Deco running in AP mode.