As far as I know using a Ni-Cd algorithm to charge Ni-MH can result in overcharge. Check BU-408: Charging Nickel-metal-hydride.

Personally I would replace that 3x AA pack with 2 or 3 li-ion cells in parallel, plus a BMS (this or that) and some TP4056 charging circuit(s). Certain TP4056 boards feature DW01A + 1x 8205A protection circuits, so two boards in parallel would handle up to at least ≈7A of load while providing 2A of charging speed.