Millisecond isomers were recently observed in ^213,215Tl. These two isomers can be interpreted by the π0h_11/2 single-proton-hole states. The 11/2^- states in Tl isotopes decrease in energy when neutrons are added to the v0g_9/2 orbital, as driven by the protonneutron monopole interaction. Based on the mechanism, this work investigates the isomerism in the southeastern vicinity of ²⁰⁸Pb with an optimized shell-model effective Hamiltonian. The 11/2^- isomeric γ-decay half-lives are predicted to be longer in more neutron-rich nuclei, potentially exceeding the β-decay half-lives. In this way, β decay may predominantly occur in the 11/2^- states, which are crucial for understanding the N = 126 waiting points and the A = 195 peak.