Colloidal nanoparticles (NPs) embody the promise of materials by design, offering atomic-level tunability for a range of applications from electronics to catalysis. Their promise has intensified the demand to scale-up NP synthesis from a scientific discovery to an engineered technology. Two limiting factors to scale-up include inherent sensitivities within conventional methods, that are amplified at larger scales, and several gaps in fundamental understanding regarding the synthetic mechanism. Hence, more rational and reliable synthesis methods (i.e., process intensification) are critical to enable NP-based technologies. This work presents efforts to produce a robust and scalable NP synthesis platform, using a heat-up method in a regime of previously unexplored high concentrations near the solubility limit of the precursors (1000 mM). In this highly concentrated and viscous regime, NP synthesis parameters are demonstrated to be less sensitive to variation and thus more reliable for scale-up. Overall, this method enables a 10-fold increase in NP volumetric production compared to conventional methods, while still producing high-quality NPs (<7% RSD). High concentration synthesis also fosters a precise reaction pathway for the isolation of high-quality (>99.9%) magic-sized clusters (MSCs). MSCs are ultra-small (<2 nm) single-sized NPs that are stable against typical growth / dissolution processes and intermediate to larger NP formation. This work demonstrates that the enhanced stability, and thus purity, of MSCs at high concentrations stems from the simultaneous production of an extensive (>100 nm grain size) hexagonal mesophase assembly that shields the MSCs from further growth. Subsequent chemical treatment of these MSCs reveals that clusters differ substantially from larger NPs. In one case, the chemical treatment triggers a reversible transformation between two distinct, stable MSCs, reminiscent of molecular isomerization reactions. Ultimately, the robust and precise nature of highly concentrated NP synthesis enables both large-scale nanomanufacturing and enhanced synthetic understanding.