Amplifier noise output against source resistance
with the noise model I have the noise would continue increasing at higher resistances , why does it start to flatten out?

The problem is that you are assuming your 2 stage amplifier has infinite input impedance. It clearly doesn't and therefore the finite input impedance value will limit the maximum thermal noise. Making your source resistance even bigger is not going to change this because, in effect, that source resistance is in parallel with the amplifier's input impedance. You'll reach a point where increasing the source resistance does not affect the noise and that is what you see.