Anode for buried galvanized steel tube
The industrial way is both coating and cathodic protection ( sacrificial anodes/aluminum).The coating greatly reduces the amperage needed from the anodes. In your case it would be so easy and cheap to drive several inch lengths of aluminum angle into the ground that I would put in several ( each one attached with a wire to the steel frame). For a limited life item ,I would not bother with coating although that is best practice. In industry they would measure soil resistivity to decide how many anodes ,but,again, not necessary for your case. The aluminum could also be used to anchor the trampoline to the ground.