The front GP-5 stock tire does wash out really easily offroad, that's not specifically your weight or the sand. I had the same issue on gravel, dirt, and grass. Also had the rear tire get squirrely slipping on those surfaces, but didn't end up stuck in anything.
I have the Shinko 241s and they addressed those issues, downside is a little bit of low speed choppiness on paved surfaces, a little more road noise, probably quicker wear on paved surfaces, and the rear does slip in the rain if you're really hooning the bike. All tires are a compromise though, gotta pick one you think will suit your riding and if it didn't then try another in a few thousand miles.