Sequential NanoPrecipitation (SNaP) is a nascent controlled precipitation process for the tunable formation of polymeric particles for drug delivery and bioimaging. While SNaP relies on the same self-assembly principles as one-step Flash NanoPrecipitation, SNaP is a two-step assembly process in which the particle core is formed during a first mixing step followed by particle stabilization in a second mixing step. Decoupling the particle assembly steps improves control over the particle structure and, as we demonstrate for the first time, expands the attainable particle size range to include microparticles. Current SNaP experimental set-ups use commercial millifluidic mixers connected in series that suffer from several drawbacks including the inability to access short inter-mixer delay times. Here, we develop a robust 3D-printed, modular mixer design that enables access to short delay times (< 25 ms) not previously accessible. We prove empirically for the first time that the inter-mixer delay time is a key parameter for particle size control and that the nanoparticle size scales with delay time in agreement with Smoluchowski's model of diffusion-limited growth. We demonstrate the formation of polymeric particles ranging in size from 160 nm to 1.2 μm. Finally, we establish the versatility and applicability of our mixer design by encapsulating fluorophores and therapeutics into particles for the first time via SNaP.