Soft magnetic mouldable composites $(\mathbf{SM}^{2}\mathbf{Cs})$ would be ideal for integrating magnetic core inductors into circuits since they can be fabricated at low temperatures and pressures. However, most of the existing $\mathbf{SM}^{2}\mathbf{Cs}$ offer either low magnetic permeability $(\mu^{\prime})$ or low quality factor $(\boldsymbol{Q})$, especially at high frequencies (> 10 MHz). To address this issue, a bottom-up approach is reported that incorporates designing the composition of the magnetic filler, coating the filler with Polydimethylsiloxane (PDMS) brush, to a state-of-the-art casting process. This procedure has enabled the fabrication of cores with an outstanding combination $\mathbf{of} \mu^{\prime}$ and $Q$. The $\mu^{\prime}$ of our cores is 40% higher than the best available data at 10 and 25 MHz which is due to the high packing of cores. Our core also offers one of the highest $Q$ at 10 MHz due to low hysteresis loss and low inter-particle eddy current loss as a result of the polymer brush insulating effect.