Daily experience demonstrates that upon mixing, oil and water quickly phase separate. While rare in pure substances, such liquid-liquid phase separation is ubiquitous in molecular mixtures as well as suspensions of nanoparticles, proteins and colloids. With few notable exceptions, surface-tension minimizing spherical droplets continuously coalesce, increasing in size without any bound, before achieving macroscopic bulk coexistence. In comparison, the phase behavior of nanoparticles or proteins dissolved in 2D fluid membranes is significantly more complex. Inclusions distort local membrane structure leading to membrane-mediated interactions that are fundamentally different from well-studied bulk interactions, yet are difficult to experimentally measure. We investigate liquid-liquid phase separation in a highly simplified system of colloidal membranes. The bulk phase separation of dissimilar rods is inherently unstable and gives way to formation of finite-sized, highly-monodisperse colloidal rafts. Using single molecules techniques we measure kinetics by which thousands of rods assemble into an isolated raft. Subsequently, we quantify repulsive raft-raft interactions and correlate them to raft-induced membrane distortions; demonstrating that particle chirality is an essential requirement for raft formation. At high densities rafts assemble into cluster crystals which constantly exchange rods with the membrane background to robustly maintain a self-limited size. Finally, we demonstrate raft polymorphism by forming supra-rafts, 2D liquid droplets with complex highly non-spherical shapes such as a beads-on-a-string polymer.