This looks like the best place to get answers on a few subtle questions
that I haven't seen addressed elsewhe

I've purchased a 10'X12' pre-fab shed as a reptile room. It's a white
pine 2X4 (mostly) frame with aluminum siding and pressure treated floor
(plywood over 2X6's).

I live in south Florida, so it's far more important to keep heat out
during the summer than in during the winter. I will be puting in a
small window air conditioner. I will be shooting for somewhere around
75-80 degrees F during the summer.

I will have 3 1/2" of space I can insulate in the walls and more in the
ceiling. Is 3 1/2" fiberglass insulation (R-13) the best I can do here?
I think I gather that using thicker insulation and packing it in will
be worse rather than better, but is there a better insulation I can
use? Everything else I've looked at seems to have a lower R value for
the same thickness.

Given that the siding is aluminum, would it make any difference to put
that reflective foil between the siding and the insulation, our would
that only help with a wood surface? How about between the insulation
and the wood panneling on the inside?

The shed is on blocks rather than a slab (maybe 6-8 inches above
ground). Would I gain anything by insulating under the floor or would I
be better off letting heat from inside radiate to the ground below,
which will be cooler than the air outside? Will it just be unworkable
to have fiberglass exposed undernieth holding in moisture (entirely
aside from the issue of animals nesting in it)? Would I gain more by
putting some sort of skirt around the perrimeter to keep hot air from
blowing under the floor?

I got the shed in all white to minimize heat absorbtion. Would it
matter much if I painted the roof with reflective silver roofing paint?

Anything else I haven't thought of?

Yes, I realize I may be going overboard, but anythything that reduces
the load on the AC and gives me more time before the room becomes an
oven if the power or the AC fails during the summer is probably worth
doing.

Also, any suggestions on how I could treat the white pine frame, in
place, to keep termites and such out that would not be toxic to
reptiles? Yes, I know the second part is pretty far afield for this
group, but I thought I'd ask...