The output power of the SI5351 is automatic set depending the requested sweep frequency, 2mA below 300MHz, 8mA above 300MHz
Similar for a manually set fixed output frequency using the freq command
You can manually control the power through console by issuing
The power is in small steps controlling the SI5351 current into the output attenuator
sets the nanoVNA again in automatic power mode
If you report output power please indicate at which power setting the output is generated.