In the present work, we have explored a series of unsaturated hexa-18-crown-6 (U18C6) ligands towards designing highly anisotropic Dy(III) based single-ion magnets (SIMs) with the general formula [Dy(U18C6)X₂]⁺ (where U18C6 = [C₁₂H₁₂O₆] (1), [C₁₂H₁₂S₆] (2), [C₁₂H₁₂Se₆] (3), [C₁₂H₁₂O₄S₂] (4), [C₁₂H₁₂O₄Se₂] (5) and X = F, Cl, Br, I, O^tBu and OSiPh₃). By analysing the electronic structure, bonding and magnetic properties, we find that the U18C6 ligands prefer stabilising the highly symmetric eight-coordinated hexagonal bipyramidal geometry (HBPY-8), which is the source of the near-Ising type anisotropy in all the [Dy(U18C6)X₂]⁺ complexes. Moreover, the ability of sulfur/selenium substituted U18C6 ligands to stabilize the highly anisotropic HBPY-8 geometry makes them more promising towards engineering the equatorial ligand field compared to substituted saturated 18C6 ligands where the exodentate arrangement of the S lone pairs results in low symmetry. Magnetic relaxation analysis predicts a record barrier height over 2700 K for [Dy(C₁₂H₁₂O₆)F₂]⁺ and [Dy(C₁₂H₁₂S₆)X₂]⁺ (where X = F, O^tBu and OSiPh₃) complexes, nearly 23% higher than those of the top performing Dy(III) based SIMs in the literature.