We highlight the lightly Br-substituted Li3YCl4.5Br1.5 as the best compromise, achieving the 'best of both end-members', with a conductivity ~200 higher than trigonal Li3YCl6 and an oxidative stability ~0.5 V higher than monoclinic Li3YBr6. This work highlights that careful optimization of composition and substitutions in the anion-framework provide prolific and rational avenues for designing the properties of future solid electrolytes.