The State adept has long lived past its usefulness, other than helping Americans that get in trouble overseas.
Outside of that, the DoD and Dept of Commerce could handle all of its functions.
As for an essential diplomat. There really isn’t one, unless you call a government employee that spends their time overseas, wasting taxpayer money, living off the taxpayer dime and living well I might add, and gaining an increasing hatred for America while embracing all the “isms” you can think of.
A non-essential one, is the one that has a made up job, has no reL purpose, buts does all the same stuff the aforementioned diplomat does.
When ISIS was threatening Baghdad, years ago, they removed all nonessential personnel. Over 1000 people were flown out of the country.
The State department ceased being a useful dept when they began representing other countries to the US instead of representing the US to other countries.