Youre over estimating weight problems. Again, 50kgs (assume 60 including the actual pulleys and ropes) across two beams. 30 kgs per beam. If you were fit and able, would you be prepared to chin yourself up on each beam? Thats about a 100 kgs per beam.
I would not use galvanised wire, galvanising is a very thin top coat over iron. A few lifts and lowers over steel pulleys could easily crack the galvanising, leading to early onset rust in shed conditions.
Very thin stainless cable, and ALL of your maintenance goes away for the rest of your life.