Pine ply is fine, kiln dried pine is fine. Plywood is heat treated to death so it is completely fine to use. I might suggest looking at Baltic birch as it is stronger and can be used in smaller thickness and at least in my area it is about the same price as fir.

i would suggest not using a wood bottom if you are thinking of UTHs. I would suggest using PVC (not expanded PVC aka PVCx just the plain with ho air bubbles in it) it is warp resistant and strong and also easier to heat through than PVCx and way easier than wood.