Remove all the end devices (the green ones). Buy and install more routing devices (light bulbs, wall sockets, maybe a couple more Sonoff sticks like the one you have).
Start by placing the routers fairly near the SkyConnect and work outwards. A Zigbee signal is weak - it can travel quite a long way in ideal conditions, but a good rule of thumb is to put them 10-15 feet apart. The aim is to create a mesh of routers placed so that any one of them can communicate with as many of the others as possible.
This map comes from the ZHA integration - the routers are the oval shapes - but you get the idea.
When you have a strong web of connections between the routers, add the end devices - ideally again not more than 10-15 feet from the nearest router.
At the moment you’re treating your router like a wi-fi extender, which is not how Zigbee works - you need a lot of routers so that messages can be passed from one to the next by different routes, depending on traffic and obstacles like walls.
There are several guides:
- Zigbee networks: how to avoid interference and get the best range and coverage
- Migrating from a Philips Hue bridge to SkyConnect
- A to Zig - a Zigbee glossary
- Not many people know that - a random collection of Zigbee trivia
- How to set up custom device handler quirks in ZHA
- Zigbee buyer's guide
- Mains powered Zigbee devices that aren't routers