This is what you can do starting from already running RT-AX86U (or something else in my case):
- connect the gateway to the existing router in double NAT and power it up
- use Ethernet or Bluetooth to start the setup process, let it update the software
- register it to your account, follow the screen instructions, get to the Network screen
- play with the entire UI to get yourself familiar with it, the UniFi system is behind your Asus, your family sees no change
- adjust DHCP range, DNS server, security features, etc. whatever you want, take your time
- power up the APs, connect them to the gateway (or switch), adopt them, let them update the software as well
- in AP settings set your known working well channels, bandwidth, power on High for now, LED light color/brightness if you want
- In main screen Settings, WiFi - make the same SSID(s) as on your Asus router with added "-1" at the end
- adjust the settings for the SSID(s) - what band(s), what security, band steering, captive portal, etc.
- play with it for few days if you want and when you're ready remove "-1" from the SSID(s)
- power down your Asus router and move the WAN cable from the Asus to the UniFi gateway
- wireless network clients will reconnect to your new APs, if some don't - forget the connection and reconnect manually
- move the wired clients to the gateway and configure whatever they need to work just as before, plus you have port VLAN option now
- you can do the switch with already created networks, assigned DHCP ranges, reservations, clients names... whatever you already configured
- your family will see up to 10 seconds reconnection time only and the life goes as usual
From single RT-AX86U to 2x U6 Mesh - you may have to dial down the power on the APs or move them further away from each other to extend the coverage area. In my tests single U6 Mesh had similar or better coverage than RT-AX86U with better throughput to the same clients at the same distance.
Overall performance - on RT-AX86U you had 4x CPU cores, 1GB RAM and 2x radios. Now you have total of 12x CPU cores, 5GB RAM and 4x radios. The gateway does routing only on separate CPU/RAM, the APs do wireless bridging only on separate CPU/RAM. You'll notice the difference.