When loading the boat I snug up the winch strap as far as reasonably possible (if I crank it too hard I’m afraid something is going to snap). But then the boat seems to move back a couple inches as pull it up the launch ramp. Is it possible that the wind strap stretches due to the weight of the boat? If so, how do I prevent this from happening?

it's the boat settling onto the trailer as you pull out - the transom settles down from a floating position and the bow comes back/up off the front winch pads

I run a separate ratchet strap from the bow eyelet straight down and wrap it around the trailer. Took me a year or two to figure that one out, does wonders to stop any bouncing.

This. But I always used my non-cranking hand to put tension on the strap until it tightened up from the weight of the boat and it mitigated the problem.

Try leaving the trailer more out of the water to put more boat weight on the strap as you crank, it's not going to take much to get it right. Once you find the sweet spot mark the strap with a bright color at a point where you'll see it (I used a silver sharpie on a black strap) and can line it up every time you load. No matter what ramp at any angle you use, once the sharpie mark is lined up to your stop point, in my case a manufacturing line on the crank housing, the boat will sit where it is suppose to be.