I wonder how things would work for you if you were using an AiMesh with dual-band Smart Connect and a single SSID. Without Smart Connect, you should be using separate SSIDs, and then roaming is not as seamless.
OE
RT-AC68U doesn't have Smart Connect as far as I know. I am using AiMesh with one SSID per Band (2.4 and 5 has different SSID).