Synthetic jet actuators are of interest for potential applications to active flow control and thermal management. Resonant piezoelectric-diaphragm-type configurations are commonly considered. Modeling of such actuators remains a challenge due to complexities associated with both electro-elastic and fluid-structure coupling, as well as potential non-linearities in both. A key metric for synthetic jet performance is the time-averaged jet momentum. Linear lumped-element modeling is an approach that has demonstrated the ability to predict jet momentum in terms of input frequency and voltage; however, it neglects nonlinearity and increasing losses at high amplitude. Full electro-elastic-fluidic finite element modeling makes the most accurate prediction but is computationally expensive for design and optimization purposes. The assumed-modes method provides an energy-based low-order model which captures electro-elastic and acoustic-structure couplings with adequate accuracy. Tri-laminar circular plates under clamped boundary conditions were modeled using the assumed-modes method. Maximization of jet momentum is considered via the maximization of surrogate device metrics: Free volume displacement, effective blocking pressure, strain energy, and device coupling coefficient. The driving frequency of the actuator is treated as a constraint in the optimization which nominally matches the fundamental acoustic natural frequency of the cylindrical cavity. Device configurations were obtained for various polycrystalline and single crystal piezoelectric materials, driven at 10% of their coercive fields in the model. The optimal configurations approximate a simply-supported circular plate with complete piezo coverage. The relative merits of individual materials were also discerned from the optimization results. The low mechanical loss factor of PZT8 enables high output at resonance, while high loss factor and low stiffness limit the utility of PVDF in this application. Due to a combination of lower loss factor and higher coupling, single crystal materials modestly outperform PZT5A.