Thanks to all for your advise, instructions and help.
Both RT-AC86U (2020 and 2019 model) updated to the latest official ASUSU 3.0.0.4.386_40451 fw and now running AiMesh via Ethernet Backhaul Mode, in AP-AP mode, DualBand Smart Connect enabled.
So far no problems (up to 20+ wired and wireless...