The directed flow (v₁) of identified hadrons (π^±, K^±, p, p̄, φ, Λ, and Λ̄) is studied in symmetric nuclear collisions (O+O, Cu+Cu, Ru+Ru, Au+Au, and U+U) at √s_NN = 200 GeV using the string-melting version of a multiphase transport model with improved quark coalescence. The mid-rapidity v₁-slope (dv₁/dy) and its charge-dependent splitting (Δdv₁/dy) between particles and anti-particles are investigated as a function of nuclear mass number (A) and collision centrality in both low-pT (0.2-2.0 GeV/c) and high-pT (2.0-5.0 GeV/c) regions. At low-pT, the v₁-slope shows weak system-size dependence, while at high-pT strong system-size dependence is found and it becomes negative with nuclear mass number, reflecting the hard-soft asymmetry in particle production. The charge-dependent splitting Δdv₁/dy reveals a striking baryon-meson dichotomy: baryon pairs (p-p̄ and Λ-Λ̄) exhibit significant splitting that grows with system size, whereas meson pairs (π⁺-π⁻ and K⁺-K⁻) show minimal splitting. The effect of final state hadronic interactions on the v₁-slope is found to be negligible confirming that it is primarily generated during the partonic phase and coalescence process. A comparison of the AMPT results with measurements from the STAR experiment at RHIC in Au+Au collisions establish the transported quark contribution as a baseline for the observed charge-dependent v₁ splitting, on top of which electromagnetic field effects must be considered.