how does a de-gas setup work or look like?
Ideally you fix the combustion leak. If it's pushing at idle and increases with load it's not really a question if it's combustion gas.
In case some missed it it's running an electric pump so it's not going to be a high rpm high pump pressure problem.
On your setup I imagine a degas system would place a canister similar to your overflow up as high as you could near the firewall. This canister would be where your pressure relief cap would be and could be a fill point. I imagine you would still likely need a overflow tank for regulations.
Plumbing the degas tank: A 3/8 hose from the outlet of the head in the highest position to a high swirl position on the degas tank. The bottom of the degas tank 1/2 hose to the inlet of the water pump.
Now combustion gas and a small portion of water flows from the high point to the degas tank, separates and the cap reliefs the gas and coolant goes back to recirculation.
Some oems will put the line on the top of the radiator on the cold side and this lets the gas settle out and cool through the rad first.
Now this would only be practical for a small controlled leak in a casting and would not control a large crack or gasket failure.
On second look is your radiator split in the middle and coolant crosses back to the passenger side?
If this is the case you have a very small flow area and maybe even the electric pump is pressurizing the hot side enough to open the cap? If be interested in cold pressure engine off while the pump was running.