2016 5.0 Coyote overheating. What am I missing?

Truck: 2016 F-150, 5.0L V8, 152k miles, recently purchased

BACKGROUND

Bought the truck, drove it 1,200 miles home. Only issue was heat only working when revving — none at idle. Found the degas bottle low, topped it off. Discovered a leaking Y-connector at the thermostat housing. Replaced it (Dorman, new o-rings). Drove a full week with no issues. Then suddenly started overheating on the highway — gauge hit red, got a warning message.

WHAT I'VE REPLACED

- Y-connector (no leaks)

- Thermostat (OEM Motorcraft)

- Radiator cap

- Cylinder head temp sensor

- Vacuum coolant fill

- Block test — blue, head gasket ruled out

CURRENT SYMPTOMS

- Overheats at idle

- Temp goes UP when revving at idle. Pretty quickly .

- No cabin heat — just room temp air

- With cap off at operating temp, coolant in the degas bottle has very minimal movement — nothing close to the active swirling/flowing you'd expect

- Both upper and lower radiator hoses are warm, upper is hot.

- No leaks, no white smoke, no sweet smell, no milky oil

- Water pump has no noise and pulley has no play

KEY DETAIL

Water pump was replaced ~15k miles ago, unknown brand. Did not get it changed at a dealership. Truck cooled fine for 1,200 miles and a week of driving before this started.

QUESTION

Is this a failed water pump impeller, a clogged radiator, or something else? The minimal coolant flow and zero cabin heat point me toward pump, but the truck was cooling fine for 1,200+ miles which makes me hesitant. Don't want to throw more money at the wrong part.

Thanks for your time.