Is the 3.5' in the shallow end water depth or pool depth? I do not think you want it shallower than 3.5' WATER depth.
Take care.
Be sure you define that the WATER depth is 3.5'. Pool depth is typically 6-8" more than water depth.
Take care.
The pool slope doesn't change from excavation to gunite.
Why do you say that? I just confirmed with my PB that it will. There will be about 9” in shallow end and 16”-18” in deep end (includes plumbing and drain). This means the depth will decrease in the deep end more than it will decrease in the shallow end which will lessen the slope. Right?
My experience viewing hundreds of builds here is that, at the dig stage, the slope is the same as the finished stage because the plumbing is dug into the ground or is just under the elevated rebar, resulting in dirt to gunite surface distance being the same in the shallow and deep ends.
I would go by what your PB is telling you as you should have no reason not to believe him. It just doesn't agree with what I've seen.
One way to check him though is to measure from dirt to the top of the guide board around the edge of the pool in the shallow end and deep end. If the difference between the two measurements is 3 1/2', something's not kosher with his explanation or math.
Just make sure your skimmer is at the downwind side of wherever the common prevailing winds are for your house. You only get one shot at it's placement, so be sure it's right!
A small tip is to put a tape measure up to your side and other in your family so you can see where 3.5 ft is. After doing that we decided in 4ft-7ft pool with similar dimensions and we are very happy with the pool and slope. It definitely is a personal preference but if your worried about to deep for the kids remember they grow by the day.
Also your depth is deeper now than the pool will actually be because they can add 9-12 inches for gunite and another inch or more for plaster and the water only goes up 6 inches below the top of the dirt
Good luck and I know you will love it either way