It looks like the usual suspects to mess with the signal, like WiFi or Bluetooth interference.
I don’t think its soley on the coordinator side like SSD or USB interference since it looks too broad.
The LQI values aren’t only weak towards the coordinator but also between the routers.
I suggest to check which WiFi channels are around that overlap with the Zigbee channel of your network. Also look with a WiFi scanner phone app whats up in the air on 2.4 GHz.
Further any kind of streaming over WiFi and Bluetooth can have an impact on the Zigbee signal.
The green lines for the battery devices may take a while to show up (also depends a bit on to which parent they connect).
The Zigbee channel 15 from your WiFi surroundings looks like a good choice.
Looking a bit closer on the MAC addresses I see some Osram/Ledvance lights?
If I remember correctly for some reason these report lower LQI values in their neighbor tables as they actually have, I this case in my bathroom for example the following OSRAM GU10 is like 50cm away from the other spots and shows much weaker signal in one direction:
Anyway, the Source Routing would be the next thing to try out, you can find it in deCONZ → Panels → Source Routing and start with the following settings:
