I was wondering if it was worth moving to two ASUS devices.AiMesh is an Asus thing. If you want to use it you will need an all Asus setup.
Afaik, you can't use 'mesh' routers between brands currently.
But with much of the advice, I've done some reconfiguration - what I can do without the factory reset - and I've been able to improve the dropped-packet problem. Turned 2.4GHz back on on the extender on a different channel and removed the sonos conflict. Backhaul still is flaky. Hoping the reset solves that, as the backhaul weirdness is new after the last firmware update that added parental controls and the new security stuff. I really am hoping the factory reset solves it.