Check your SiLabs Mutliprotocol AddOn Logs. Are there any errors with it?
If not, restart Z2M AddOn and look back in the SiLabs logs and you should see a line in the logs, something like “client connected”. If you do, then that should mean that Z2M has connected to SiLabs Zigbee stack. It may be quickly followed by something like “client disconnected” which means Z2M gave up and disconnected and it means the problem is somewhere in the SiLabs and or SkyConnect.
BTW, I think your Zigbee2MQTT config is OK, at least for connecting to SiLabs.
Also, the reason I moved away from SiLabs is that it would disconnect every so often with SkyConnect, plus the Z2M AddOn has to wait for SiLabs AddOn to stabilize after a restart which caused problems similar to what you are seeing.