  Dense ensembles of spin qubits are valuable for quantum applications, even
though their coherence protection remains challenging. Continuous dynamical
decoupling can protect ensemble qubits from noise while allowing gate
operations, but it is hindered by the additional noise introduced by the
driving. Concatenated continuous driving (CCD) techniques can, in principle,
mitigate this problem. Here we provide deeper insights into the dynamics under
CCD, based on Floquet theory, that lead to optimized state protection by
adjusting driving parameters in the CCD scheme to induce mode evolution
control. We experimentally demonstrate the improved control by simultaneously
addressing a dense Nitrogen-vacancy (NV) ensemble with $10^{10}$ spins. We
achieve an experimental 15-fold improvement in coherence time for an arbitrary,
unknown state, and a 500-fold improvement for an arbitrary, known state,
corresponding to driving the sidebands and the center band of the resulting
Mollow triplet, respectively. We can achieve such coherence time gains by
optimizing the driving parameters to take into account the noise affecting our
system. By extending the generalized Bloch equation approach to the CCD
scenario, we identify the noise sources that dominate the decay mechanisms in
NV ensembles, confirm our model by experimental results, and identify the
driving strengths yielding optimal coherence. Our results can be directly used
to optimize qubit coherence protection under continuous driving and bath
driving, and enable applications in robust pulse design and quantum sensing.
