External Sonnet 550 eGPU not providing enough power for MacBook Pro (15-inch, 2018)
If you have a new box that you're still able to return, you could consider buying the 650 model instead of 550 model that you currently have. The 650 models supports charging the laptop with up to 100 watts, whereas the 550 model only supports 87 watts.
In regards to switching power source and your comment regarding that you think Apple incorrectly states that the source with the most power is used:
The 2018 version of the 15&quot; MacBook Pro comes with a 87 watts charger. So it is actually rates to supply the same amount of power as your Sonnet box. Therefore I don't think you can expect the laptop to switch to it when you connect it.
It is only the newer models from 2019 and on that comes with a 96 watts charger.
You could consider buying the newer 96 watts MacBook Pro charger to ensure more power is supplied to the laptop, but you'll still have to plug in two cables this way.
There's no command in Terminal that allows you to choose a different power source for charging.