You don’t have enough routers. If you have 75 devices, I’d suggest that as many as half of them need to be routers.
Your LQI numbers are very low. They should be in the 100-150 range at least, ideally higher. To achieve this, you need more routers - lightbulbs, smart plugs, wall sockets, whatever.
The optimum distance between devices is 10 feet, not 10 meters. Yes, the signal will travel much further in good conditions, but that’s not the point. The point is to build a robust mesh that will self-heal when there are problems. You need more routers.
Stop obsessing about the map. It’s a snapshot visualisation and it will be different every time you generate it.
YOU. NEED. MORE. ROUTERS. 
Edit: Have you read this?