Recurring radiator issue, blown head gasket?

Hi,

I made a prior thread a while back concerning this issue but wanted to make a new one with new insights I've found. The issue is as follows:

The coolant level in my 2007's radiator consistently stays about three inches below the filler neck. It does not decrease below this height, nor will it pull in coolant from the overflow. I have driven it as such for months to confirm this. Upon refilling the level to its proper height to the filler neck, putting on the cap, and starting the engine, coolant will begin to quickly gush out from just beneath the cap after only ten minutes, when the engine temperature reaches the first notch on the temp gauge. I have been through four different radiator caps and none have stopped this behavior.

I have verified that the hose from the neck to the overflow tank is not clogged, and used a ziptie to firmly seal it to the horizontal valve in the filler neck. I can watch it, and it does not leak from the hose. I have rented an Autozone pressure test kit and pressurized the system with the engine off at 16psi. It holds pressure just fine. The mating surface between the cap and gasket is clean, tight, and free of nicks, and the caps are not missing their gasket (see photos).

This troubles me, because I am beginning to suspect there is a greater issue here. I find it hard to believe that coolant is gushing out at the rate it is after the engine barely gets to operating temperature. Part of me wonders if there are exhaust gases somehow escaping into the cooling system, creating excess pressure beyond 16psi that would cause this behavior. But if this is a case of a bad head gasket, shouldn't the engine be burning up coolant over time? It's never run dry or fallen below the same three inches under the filler neck, and I'm not blowing white smoke out the tailpipe either.

Of course I could continue to drive it this way, as most people would. But this small annoyance has been bothering me and I'll be damned if I can't figure out the issue. I've attached a photo of the cap and cap mating surface (AFTER topping off fluid). Any and all insights are welcome. Thanks.