Adiabatic demagnetization refrigeration (ADR) based on frustrated rare-earth magnets provides a promising route toward helium-free sub-Kelvin cooling, while balancing magnetic entropy and interaction scales is essential for optimizing the ADR functionalities. Here we investigate the low-temperature magnetic and ADR properties of the alkali rare-earth double phosphates K₃Ln(PO₄)₂ (Ln = Yb, Gd), which crystallize in a monoclinic P2₁/m structure containing distorted isosceles triangular lattices of rare-earth ions. Magnetization and specific heat measurements confirm the absence of long-range magnetic ordering down to 0.1 K in both compounds. Direct quasi-ADR measurements demonstrate strongly contrasting refrigeration behaviors: K₃Yb(PO₄)₂ reaches a minimum temperature of 29.8 mK from an initial state of 2 K and 6 T, whereas K₃Gd(PO₄)₂ exhibits extended hold times below 150 mK associated with its larger magnetic entropy reservoir. Crystal-electric-field calculations confirm a J_eff = 1/2 ground-state doublet in K₃Yb(PO₄)₂, while thermodynamic analysis based on magnetic entropy and interaction scales accounts for the distinct ADR performance of the two compounds. These results identify K₃Ln(PO₄)₂ as a chemically robust platform for rare-earth-tunable solid-state ADR.