Hi,
I’d suggest a read of this Zigbee thread for a few ideas on the difference between Routers and End Devices, and also several nuggets of real-world experience with good and bad devices (including some that can make a mesh unreliable!):
TL;DR - some devices don’t work with others; add more mains powered devices to create routers and extend the mesh (also reduces battery drain). Use the mesh network map to look for poor (red) paths under the ZHA Visualisation tab.
ZHA seems to be going through a bad patch at the moment with less tolerance of ‘naughty devices’ like IKEA remotes (which need regular reset, and re-adding; re-interview always fails removing parts of the pairing like battery status).