Hi XXXXXXXXXX;
The wood frame is not needed. The sand is. You will need about 2-3 inches under the pool wherever there is contact with the ground.
As for the low points just fill with moist dirt and compact it well with your feet. Add about an inch of extra sand there.
Use what they call 'Clean Masons Sand'
Anything else may have small stones that can tear the liner and cause a leak.
You will need about 2 tons (average scoop from a supply yards tractor is equal to one ton, as well as being pretty close to a square yard) to do the job right. One may work but it is better to have extra than not enough in this case.
Please post any other concerns here.
Best, THS
Jack of all trades Master of a few.
#1 Home Improvement Expert. Over 30 years experience in construction, and home repairs