FAST uses a constant-time-step integrator. I’m not sure I understand what your concerns are about things staying constant. To best see what is happening, set DecFact to 1 so you see every step.
DTAero should be an integer multiple of DT. A rule of thumb I use is that I like to have at least 200 time steps per revolution. If your turbine is variable speed, I’d use the fastest likely rotor speed to compute that.
I also recommend you start at some large time step and do runs with smaller and smaller time steps until the answers quit changing appreciably. Of course, this number you get will depend on the case you run. For instance, you will need smaller time steps higher wind speeds if you have a variable-speed turbine. As you enable/disable degrees of freedom, this will also have an impact on the choice of time step.
There is no way to control integrator precision.