You can think of the ground as one plate of a capacitor, the atmosphere as the dielectric, and the elevated insulated plate as the other plate of the capacitor. The elevated plate collects a charge on one side of your device and the opposite charge from the ground is attracted to that collected charge in your device thus causing a current to flow.

Grounding devices need to have as little resistance as possible. That's why copper is better than iron. I suspect your ground rod was steel rather than iron and that is even worse than iron for conducting electricity. Most ground rods are copper coated steel rods and since copper is a better conductor than steel the charge flows in the copper. The steel core is there to provide stiffness to allow driving the rod into the ground. Solid copper, being relatively soft, would just bend nto to mention very expensive.

Ground rods are typically eight to 10 feet long. The more surface area exposed to the soil, the more charge will be collected. Jes Ascanius later used a large plate buried a couple of meters deep and connected to three insulated wires that ran to the surface which were all connected together at the top and then connected to his device, the connection and above ground wire being insulated also. I don't remember the size of the plate but it was roughly 1 1/2 X 2 meters. Again, larger is better.

The charge collected in this kind of system is weak and it will neutralize with the opposite charge in the air so everything above ground needs to be insulated to prevent that. You want the collected charge to get to your device rather than neutralize on the way to your circuit.

Charges from the atmosphere can penetrate the ground so it will have both positive and negative charges in the top three or four feet. The deeper you bury your ground rod/plate the more isolated the charge that it is collecting will be and that's the whole point in this system - charge separation. If you use ground rods you need to insulate the top three feet or so and then use a spray on insulation to isolate the connection to the ground rod that goes to your circuit. I initially didn't realize this and used the ground rods without insulating the top part. When I used heat shrink tubing on the top three feet and sprayed Plasti-Dip on the connection my charge collection improved substantially even with less surface area of the ground rod exposed to the soil.

The same applies to the elevated plate. It needs to be insulated so the collected charges don't neutralize on the surface before they flow to your device. The higher the dielectric constant of the insulation used on the elevated plate, the more collected charge will make it to your device. The insulation provides the charge separation at the elevated plate and "dielectric constant" is a measure of how well a material will do that.

Be aware that any exposed corners or sharp points on your elevated plate will ionize the air around that point. The ionized air around the point is a better conductor of electricity than the air so any lightning will find an easier path to ground through the ionized air and strike that point. That's exactly how a Franklin lightning rod with its pointed end works. So be sure to completely insulate the plate and its connections also. This is also why you do not want to use the ground connection to your home. Even if it is well insulated today, weather, birds, etc can damage the insulation and cause a risk of lightning strikes.

You might want to split your 50mm pipe and flatten it so you get more ground contact on both sides of the copper rather than just the outside of the pipe. Then attach an insulated wire to the center of the plate and run that to your circuit.