Thanks to Extinct, I have an update:

I had thought I was using the correct pressure testing cap, but it wasn't the right fitting - the manual said it was and it fit, but it didn't seal properly. There was another that fit and sealed. I was able to pressurize the system (went to 20 PSI).

The thermostat housing appears to be the problem - it looked like the seam was failing and causing a leak. The pressure slowly decreased. It didn't look like there were any other leaks, but there's definitely a leak in the thermostat, so I'll start with the inline thermostat modification and at the same time, I'll replace the expansion tank and the water pump - all the parts have been ordered, including some transmission cooler hose since I'm going to bypass the throttle body heater plate after all and change the rigid line the runs from the radiator to the expansion tank, which doesn't route properly around the radiator.

It's been about 4.5 years since the coolant was drained. I've heard all sorts of different things about which coolant to use, but I'd lean toward using the green stuff I'm used to seeing in there and adding when it's a bit low. Since I don't do this often, I'd like to get some good juice in there.

I have a replacement power steering pump to install as well while I'm in there, so this is going to be a lot of work for me, but it should be a good preparation for the head gasket work, which I'm lucky to hear isn't critical right now.

It's time for an ultra gauge as well - leaning toward the wired type, but welcome to any input.

I think the vehicle will be very happy after all of this work - Thanks everyone