Swimming Pool on Elevated Deck

[XR250]

I have a 5'-4" deep, 8ft x 16 ft. swimming pool that is to be sunken into an elevated wood deck. The deck is 20 ft. in the air. This is really just a giant hot-tub as far as I can tell.
The pool supplier's info suggests that it should be supported with a concrete slab. I talked to the tech rep and he said it needs to be level within 1/4". Most hot tub manufacturer's say the same thing about their tubs.

The contractor wants to support the pool on a wood platform supported by (12)8x8 timbers (about 14'-6" long) (3) rows
I can make a wood structure using 2x12's and keep the long term deflection down to under 0.1" pretty easily.
I am concerned that the contractor cannot build a deck to within the required tolerances - especially after shrinkage occurs. One idea I had was to pour a 4" slab on top of the timbers on PT plywood so it can be leveled out after the wood construction.
Another idea, is to do a perimeter wood frame and use a composite form deck in lieu of the floor joists. I could lag the decking directly into the top of the timbers.

I like to get some feedback on these ideas.

Thanks

 

[r13]

I think if you can make the 20' deck stable, then you can do everything to meet the leveling requirement. Why they emphasis the 1/4" tolerance?