Welcome to J.L. Here are a couple of observations- some of us (me) are not too smart, so you need to be very specific when you describe what you are doing. For instance-
“I was getting a lot of coolant pumping out of the center pressure cap after highway driving.”
Was the engine still running, or not? Was the cap on the header tank in place? (Header tank is what you are calling the “overflow” tank, I think.)
First, the cap on the crossover pipe fill tube will not relieve pressure. You could have a 7lb cap there, or a 20psi cap. It doesn’t matter. Pressure is released by the cap on the header tank.
If you went for a good drive, and then took the center fill cap off, expanded hot coolant will go everywhere. Bad idea.
Try this. Elevate the front of the car a couple of inches. Loosen, but do not remove the bleed plug. With both radiator caps off the car, without starting the engine, fill the cooling system starting with the header tank. When the header tank is FULL, cap it. Then ignore it. Continue filling the center fill pipe until it is also full to the top. Again, do not start the engine!
Now unscrew the bleed port plug. You should see the water level drop in the center fill tube. Slowly add more coolant to the center fill tube, and you should see coolant coming out of the bleed port. Once you have decent flow there, put plug back in. Top off the center fill port to the top. Cap it.
Now you can worry about whether there is an air pocket in the heater by running the engine with the heater “ON”.
If your air purge system is working as it should, and you know about the “Real” overflow container behind the LHS front wheel, you should make progress. If you have read Kirby Palms “Book”, that would help you figure things out, too!