I haven't done any channel optimisation so far, I probably should do. I have the P version of the Sonoff Zigbee dongle, I haven't tried the E version.

I've also been careful with picking router devices. I've tried to avoid router devices that have poor reported compatibility with Aqara. There is a page/thread somewhere online where people were compiling lists of devices that do or don't play nice with Aqara. IKEA devices apparently work very well with Aqara, I've been using their smart plugs wherever I can.

My Zigbee network also improved a lot when I set up some IKEA plugs in the loft. My house also has thick walls, but it seems Zigbee signal can propagate more easily through ceilings/floors.

Using quality brand batteries also seems to help a bit, at least from a battery life point of view.