Triangular-lattice quantum magnets with strong spin-orbital coupling constitute a fertile playground for realizing unconventional magnetic ground states. Here, we report the synthesis, crystal structure, and magnetic properties of a new family, ${\mathrm{Ba}}_3{\mathrm{REBiPbB}}_4{\mathrm{O}}_{13}$ ($\mathrm{RE}=\mathrm{Pr}$, Nd, Gd–Yb), where perfect triangular-lattice layers of ${\mathrm{RE}}^{3+}$ ions are well separated ($>13Å$) along the $c$ axis in an AAA-type stacking, resulting in an exceptionally low magnetic ion density. No evidence of long-range magnetic ordering is detected down to 0.4 K for $\mathrm{RE}=\mathrm{Gd}$ and Yb members and to 1.8 K for the others, despite dominant antiferromagnetic interactions. Quasi-adiabatic demagnetization measurements show that the Gd compound reaches a minimum temperature of 69 mK, while the Yb counterpart cools down to 26 mK from 2 K under an initial field of 9 T, among the lowest temperatures reported for similar conditions. Notably, the Gd compound exhibits two distinct temperature minima in the cooling profile and a broad peak in specific heat at approximately $0.19\mathrm{K}$, indicative of a cooperative magnetic ground state without long-range ordering. In contrast, the Yb member likely remains paramagnetic down to its lowest cooling temperature, 26 mK. The diversity of RE ions in triangular-lattice ${\mathrm{Ba}}_3{\mathrm{REBiPbB}}_4{\mathrm{O}}_{13}$ provides a low-magnetic-ion-density platform for exploring spin-orbit-driven exotic quantum magnetism, as well as potential applications in sub-Kelvin magnetic cooling.

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• Frustrated magnetism
• Magnetic anisotropy
• Magnetic refrigeration
• Magnetocaloric effect
• Quantum spin liquid
• Spin-orbit coupling
• Rare-earth magnetic materials
• Triangular lattice