Filling in a sloped portion of backyard

5 degree slope don't sound like much... Maybe over a 14 ft run its significant.

I mean cheap way would be backfill from "bottom area" (furthest from house) back toward house and level it. If you have, put stake at top and bottom. Tie string ground level at top stake and then tie several inches up (eyeball level) bottom stake taut.. Then take level and walk down the string and adjust. Then you know how much sand you would need to level off. But it will degrade over time due to rain, water out pool etc.

Personally I would do in reverse. I'd dig out the upper part to level off with bottom and use minimal amount of sand to finish off.

I had an extension to my patio poured. Only a 14 by 16 area but even they pulled out surveying equipment to make sure it was level. I was like daaang these dudes are good lol