  State-of-the-art hybrid perovskites have demonstrated excellent functionality
in photovoltaics and light-emitting applications, and have emerged as a
promising candidate for exciton-polariton (polariton) optoelectronics. In the
strong coupling regime, polariton formation and Bose-Einstein condensation
(BEC) have been demonstrated at room-temperature in several perovskite
formulations. Thermodynamically, low-threshold BEC requires efficient
scattering to $k_{||}$ = 0, and many applications demand precise control of
polariton interactions. Thus far, the primary mechanisms by which polaritons
relax in perovskites remains unclear. In this work, we perform
temperature-dependent measurements of polaritons in low-dimensional hybrid
perovskites with high light-matter coupling strengths ($\hbar \Omega_{Rabi}$ =
260$\pm$5 meV). By embedding the perovskite active layer in a wedged cavity, we
are able to tune the Hopfield coefficients and decouple the primary polariton
relaxation mechanisms in this material for the first time. We observe the
thermal activation of a bottleneck regime, and reveal that this effect can be
overcome by harnessing intrinsic scattering mechanisms arising from the
interplay between the different excitonic species, such as biexciton-assisted
polariton relaxation pathways, and isoenergetic intracavity pumping. We
demonstrate the dependence of the bottleneck suppression on cavity detuning,
and are able to achieve efficient relaxation to $k_{||}$ = 0 even at cryogenic
temperatures. This new understanding contributes to the design of
ultra-low-threshold BEC and condensate control by engineering polariton
dispersions resonant with efficient relaxation pathways, leveraging intrinsic
material scattering mechanisms for next-generation polariton optoelectronics.
