The thing that really helped me was using my philips hue bulbs with my zigbee device directly instead of through the hue hub. That made the mesh network extend to the entire house instantly, because the lights have constant power and act as repeaters automatically.
I'm using an Electrolama zzh with an extension cord and it works perfectly, because there's essentially 1-4 repeaters in every room.
How are the Trådfri bulbs? I haven't tried them out yet. It'd be good to have a cheaper option.