The interplay of geometric frustration, crystal electric field effects, and Ruderman-Kittel-Kasuya-Yosida (RKKY) exchange interaction can give rise to complex and intriguing magnetic behaviors in rare-earth-based triangular lattice intermetallic compounds. In this work, high-quality single crystals of LnNiAl₄Ge₂ (Ln = Gd, Tb, Dy, Ho, Er) were successfully grown using the Al-Ge self-flux method, and their crystal structures, magnetic properties, and specific heat were systematically investigated. X-ray diffraction analysis shows that all compounds crystallize in the centrosymmetric rhombohedral structure with space group R¯ 3m, where the rare-earth ions form a two-dimensional triangular lattice within the ab plane. Magnetic susceptibility and specific heat measurements reveal that GdNiAl₄Ge₂ undergoes two successive transitions at 15.35 K and 8.77 K, TbNiAl₄Ge₂ has a single transition at 8.83 K, DyNiAl₄Ge₂ at 8.07 K, HoNiAl₄Ge₂ at 5.83 K and 4.29 K, whereas ErNiAl₄Ge₂ shows no magnetic order above 2 K. From Gd to Ho, the magnetic transition temperatures decrease systematically with decreasing rare-earth ionic radius, while for Er the magnetic ordering temperature is likely below 2 K, and the overall trend is in agreement with that of the de Gennes factor, indicating the dominant role of the RKKY exchange interaction in the magnetic behavior. The isothermal magnetization curves measured at 2 K reveal that GdNiAl₄Ge₂ is nearly isotropic with a subtle hysteresis loop near 0.42 T, while the other compounds exhibit distinct magnetic anisotropies due to crystal electric field effects: TbNiAl₄Ge₂ shows strong easyplane anisotropy, DyNiAl₄Ge₂ and HoNiAl₄Ge₂ display complex XXZ-type anisotropy, and ErNiAl₄Ge₂ exhibits easy-axis anisotropy. Furthermore, rich field-induced metamagnetic transitions are observed: TbNiAl₄Ge₂ shows two transitions in the ab plane at 0.40 T and 2.00 T; DyNiAl₄Ge₂ exhibits two transitions in the ab plane at 0.65 T and 1.25 T and two along the c axis at 0.62 T and 3.92 T; HoNiAl₄Ge₂ displays two transitions in the ab plane at 0.35 T and 0.55 T and one along the c axis at 0.34 T; ErNiAl₄Ge₂ shows a single transition in the ab plane near 9.25 T and an anomalous high-field feature along the c axis likely related to spin reorientation. Additionally, broad CEF-derived peaks appear around 85 K along the c axis for TbNiAl₄Ge₂ and around 30 K in the ab plane for ErNiAl₄Ge₂. The single-crystal magnetic properties of TbNiAl₄Ge₂, HoNiAl₄Ge₂, and ErNiAl₄Ge₂ are reported here for the first time. This study provides systematic experimental results for further studies to understand the interplay among geometric frustration, RKKY interaction, and crystal electric field effects in metallic triangular lattice systems.