Hola Jay -
xxx
First, for the full FAA ground instructor certificate, you are looking at 3 written exams.
(1) Fundamentals of instruction - which by the way is same as the test for CFI Fundamentals.
Should you have passed that test, and later become CFI, no need to do that test again.
(2) Ground Instructor (Basic) which permits to teach PPL subjects.
(3) Ground Instructor (Instrument) which permits to teach IR subjects.
Having the (2) and (3) described here, you get "Advanced" privileges as well to teach CPL.
xxx
Should you decide to only seek the Ground Instructor (Advanced), then you have 2 written tests.
When you hold passing grade (70%) on the 2 or 3 tests above, the FAA issues the certificate.
No oral exam to take. You could even teach FAA ATPL students.
xxx
Being a good ground instructor is a specialty.
You can teach classrooms, or individual students on a one-to-one basis.
I have to tell you that CFIs are often weak at "teaching" aviation.
After all, nowadays, CFIs are often young (inexperienced) pilots theirselves who "build time".
They prefer to be "paid" for flight hours" than "briefing" (for free at times) on ground.
xxx
I do not know the current "market" for employment as such.
You could help "wannabees" their first lessons about how an airplane is.
I did that from age 17... teaching aero-club members in classrooms.
Was basic PPL air law, or navigation, or meteorology, Cessna 150, etc...
Was very little money they were paying me, but helped me for my ratings.
Forced me to study well myself, to answer their questions.
But was perfectly legal... and the flight instructors kept an eye on what I was teaching.
xxx
You could specialize yourself as a type rating classroom instructor.
Teach "systems" for Citation or Learjet airplanes, even airline equipment.
When I became an airline pilot, I soon became instructor, in classrooms then simulators.
To give you an idea, I often do airline instruction on contract base.
Of course, it is classroom, simulator and line training as I am a pilot.
I generally charge my "full" rate per hour for instruction/line training in airplane.
I charge "50% of that rate" in simulators or cockpit procedure trainers.
In the classroom, I do charge "25% of my full rate"...
And of course, a "minimum" per day...
And I teach subjects that are "my specialty"...
xxx
A few pilots I know do contract training as such after retirement.
Some others teach classroom or simulator after having lost their medical.
xxx
Best of luck to you.
Happy contrails