@Nobby 

If all your Deco units are wired, it does not matter if they are tri-band or not. You are not benefiting from tri-band, because third band is used only for WiFi link between Deco units.

Deco X90, X60 and X20 support WiFi6. Deco M9 Plus does not. Adding single Deco X90/60/20 unit to Deco mesh won't make it all support WiFi 6: wireless devices connected to Deco M9 will have same Internet speeds they do now. Only devices connected to your new X90/60/20 unit might see faster speeds.

Which means, in your case, as long as you keep all your Deco units wired, as you should if you can, spending money on X90 is unnecessary. X60 or even X20 can do.

In case you want to have WiFi 6 coverage all over your house for faster WiFi speeds everywhere, you'll need to replace all M9s with X20s or X60s. Depending on your house layout, X20s may be enough, but if your budget allows it I would suggest X60s.