You can experiment with your C8 ZigBee power levels, antenna orientation and hub location in an effort to find the best reception between the three(4?) devices.
Or... add some repeaters to help with your mesh.
Tuya USB's if you prefer something small.
IKEA plugs with this driver if you have space for them.
For the Sonoff Temp/hum sensors you can use this driver to monitor the healthStatus of the devices.
