Disregard any recommendations for part changes until you find out the cause. Smart people dont pay people to swap parts, they pay people to fix the broken part. Dont be a part swapper.

You can pressurize the system which will tell you if you have an issue. Local parts stores will loan you the tools.

I have an extreme hill behind my yard that leads into the river. I park on that hill when I bleed my truck. Its quick, no issues.

Putting the truck on jack stands is not high enough imo.

Find a hill, park it front up.

Let it idle. Turn the heat on. Watch the gauge. You can do things like step on the gas, squeeze the hoses which will help flow.

If you really need insurance, put your stat in a pot of water and verify it is opening.

Borrow or buy a motor scope which is a small camera that you can feed into your radiator or heater core to see what the internal condition is. It will help you determine if its clogged.

Use super flush.

Use garden hose, back flush.

Read your spark plugs, smell your spark plugs. Stick the microscope in the combustion chamber and see what it looks like in there...

What temp does it go to when it idles?

Add the coolant straight into the tee instead of the tank. I use a turkey baster.

Yes +1. Add coolant directly into T when the engine is cold. Replace screw before you start engine.

I had the same problem. Check your expansion tank cap, my seal was sliced in half. Even if yours doesn’t look bad they are only 20 bucks at advanced auto and that’s what did it for me.

I forgot to mention I replaced that too along with all the other project items.

Throttle Body Heater Plate still installed? Common place for air to enter the cooling system if it’s seeping. I still believe keeping it installed on a D2/P38 = disaster waiting to happen.

Yep, it's still installed. All that to say, I'm going to do a pressure test and maybe even make a little write-up for it. The test should identify that as a leak, yeah?

Yep if it’s leaking you’ll know it with a pressure test. Just don’t go over 16PSI.

Sounds like you've done quite a lot already. Maybe double check that all hose clamps are tight. Outside chance- had a hose once that it turned out would balloon under load and allow that little bit of air in. Not likely, but it happens. Keep at it though, you'll get it.

perhaps you are sucking air into the heater core via a flaky connection somewhere. Check all the connections and clamps for total continuity. soapy water in a spray bottle may help though that is generally better for pressure-side discovery.