Choosing minimum sampling frequency of a filtered signal
As you say, you should select a sampling frequency at least at twice the largest frequency you wish to keep.

What you'll do is called downsampling: going from a higher to a lower sampling frequency. This process usually starts by filtering, so there is no energy above the new Nyquist frequency, and any aliasing is negligible.

Many numerical programs, such as Matlab, include the filtering step in commands such as downsample and resample. In any case, you should always look at the spectrum before and after downsampling, and make sure the process meets your requirements.