"MRW"
How did you get 3 mV/m? I tried 300 mV/m / (sqrt(1kW - 20W)).

Other things equal, the far field varies by the square root of the power
change.
If 1 kW into a given antenna system produces 300 mV/m at 1 km, then 0.1 W
into that same system produces SQRT(0.1W / 1000W) x 300 mV/m at 1 km,
which is 3 mV/m.

Are there any existing resources that talk about how much attenuation
can each ground type contribute?

The FCC charts probably are the best resource. Surface wave propagation
losses are dependent on frequency as well as ground conditions.

I don't have the experience background, yet. But in your opinion, is a
field strength of 1mV/m the absolute minimum for decent AM reception?
Also, is the fact that AM more prone to interference the reason why its
field strength requirements are higher than FM?

A good AM receiver in the absence of natural/man-made interference can
produce useful (but not highly "competitive") performance in fields of less
than 100 µV/m.

At a given value of received field strength, an FM broadcast is received
with less noise than AM because of the lower noise level at VHF frequencies,
and due to the ability of an FM receiver to the reject the a-m noise that is
present.

RF