I did the same for my outsourced side-project - I went full waterfall.
The spec turned out to be 250 pages long (without pics) and took almost a year to fully develop from the conception stage. In addition to this, I hired a designer to develop an additional design spec for the UI.
So far, this worked great. My total time spent on this project since the moment I gave the spec to developers is about one workday in total. The questions from the devs are mostly trivial, and solvable in an hour or less.
The spec turned out to be 250 pages long (without pics) and took almost a year to fully develop from the conception stage. In addition to this, I hired a designer to develop an additional design spec for the UI.
So far, this worked great. My total time spent on this project since the moment I gave the spec to developers is about one workday in total. The questions from the devs are mostly trivial, and solvable in an hour or less.