I think what you have is called a driven well, meaning there was a sand point put on the well pipe and it was just driven into the ground by pounding the pipe down a few inches at a time. You have a checkvalve between the pump and the wellhead which prevents water from just flowing back down into the well when the pump cuts off. We don't have those kinds of wells around here, so I'm not real familiar with them. This is my best guess.
First remove the pressure gauge and prime the pump through that hole. Pour in water until it overflows. I don't think it will take much since you have a checkvalve a foot or so from the pump. Replace the gauge and give it a shot. It should begin to generate pressure. Be sure to use teflon tape on the threads of the pressure gauge. Airtight seals are important. Try that two or three times if needed.
If priming doesn't seem to do the trick, I'd remove the jet and make sure it is not clogged. I notice you are using filter cartridges. You must have some problems with foreign material in your water, so it's possible that the jet or the pump impellers are getting filled with trash. Check them both. If you're not sure how to do that, check out your pump manufacturer's website. You can probably find a parts schematic showing how to get to both. It is not a huge chore.
If that doesn't work, then this is what I'd do if it was my well. I'd remove the large threaded cap over the wellpipe and fill that with water. It is at this point that my non experience with these wells hinders me, but I think that pipe should be full of water. If it's not, then fill it to the top and then replace the threaded cap. I would think that pipe needs to stay full of water for your pump to work, the idea being that when the pump shuts off, suction holds the water in the pipe rather than just letting it run back into the well. The checkvalve above ground is responsible for that. Turn your pump back on and try it again.
One other point. If this is a driven well with a sandpoint, then those points also can get clogged with fine sand to the point that it will not let water enter. That could explain the gradual reduction in water volume you have experienced. Still, even at that the pump should produce at least some pressure, and that seems to be what you are lacking.
And, of course, the pump could be going out.
Let us know how it goes. BTW, your switch and p gauge seem to be working fine. They are not the problem.
This video about priming is useful, especially the part about using the valve to help out. Ignore the material about the disconnect.
https://www.youtube.com/watch?v=_SsTj8ZtGLo