This will raise the level of both marching and playing much faster than always marching & playing at the same time. Gradually the separate skills become automatic as they are synthesized.
The Front Ensemble plays all the time since they have no marching responsibilities. Early season use of the amplified metronome will be a big help here.
Further, one specific skill or problem should be addressed at a time. It might be dictated by the director – “focus on a straight line path.” It might be different things for everyone in the band. What is helpful is for the director to say “focus on ONE music (or visual) event that is the biggest problem for you.” Give them 5 seconds to think about it and then go.
Focusing on ONE thing at a time – Music OR Visual – is the most efficient way for the students to become comfortable at marching and playing (or spinning) and achieve their potential.