I wouldn't worry about the soft spot too much, it could just be the foam supports underneath the tub itself and not the floor. Even if it is the floor, that's probably within your skill-set to fix if you can manage a circular saw and swing a hammer. I hired a contractor to install a shower for me in a new bathroom because I don't have the plumbing skills or time. He outsourced the plumbing, electrical, and did the, framing, tiling, and everything else himself. The shower alone took them about a week, one of the main issues he encountered was keeping the tile lines straight. His tile saw wasn't exact enough and the cuts were ever so slightly off which caused compounding problems.