No Heat - Replaced multiple components

I have a 3rd Gen Prius which has done 300k+ miles. The car has recently started to give a few issues.

The main issue is that I currently have very limited/no heat coming through the vents.

I have checked and replaced the following:

1. Tried to flush heater core with hose pipe. No change.
2. Replaced water pump. No change.
3. Replaced thermostat. No change.
4. Ended up cleaning intake manifold and throttle (probably has no effect on heat but I had it open so why not!)

The last thing I can think of is the heater core itself. There used to be a step by step video on YouTube showing how to take off the dashboard etc. Can't seem to find it now. Anyhow this is the last resort.

Does anyone know what else it could be apart from the heater core?

Could I use a coolant flush chemical like liquid moly to perhaps agitated any debris within the heater core thereby returning heat?

At a crossroads and not sure what to do.

Note:
If anyone ever cleans their intake manifold and gets either a P0121 or P0443 code, then check all the clips and hoses are connected to the throttle body. Had to remove the air intake assembly twice because I kept on missing things.