Sadly I have set my 2 AX86Us up as an AP and AiMesh node pair tonight running the latest stock firmware. I can get the clients to initially connect, but after a few minutes they migrate to the AP and dislike the mesh node. Therefore I now think we still need to wait for ASUS to fix the issues in the stock firmware before even considering whether RMerlin can merge them into his own code.
Next step is to run both as APs and prove that they work fine individually, but that means relying on roaming assistant.
It is the clints decision where to connect. Many clients will attempt to the last AP they connected to before checking if there is a stronger signal. This is true even if the client is power cycled.