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 K${}_3$$Ln$(PO${}_4$)${}_2$ ($Ln$ = Yb, Gd), which crystallize in a monoclinic $P{2}_1/m$ structure containing distorted isosceles triangular lattices of rare-earth ions. Magnetization and specific heat measurements confirm the absence of long-range magnetic ordering down to 0.1~K in both compounds. Direct quasi-ADR measurements demonstrate strongly contrasting refrigeration behaviors: K${}_3$Yb(PO${}_4$)${}_2$ reaches a minimum temperature of 29.8~mK from an initial state of 2~K and 6~T, whereas K${}_3$Gd(PO${}_4$)${}_2$ exhibits extended hold times below 150~mK associated with its larger magnetic entropy reservoir. Crystal-electric-field calculation confirms a $J_{eff}=1/2$ ground-state doublet in K${}_3$Yb(PO${}_4$)${}_2$, while thermodynamic analysis based on magnetic entropy and interaction scales accounts for the distinct ADR performance of the two compounds. These results identify K${}_3$$Ln$(PO${}_4$)${}_2$ as a chemically robust platform for rare-earth-tunable solid-state ADR.