What should I do about the floor for a outside metal shed / ventilation
The plastic is good to prevent growth from taking over the shed floor. You're now paying the price for not taking the time to do proper ground prep. This is going to be some additional work, but think about adding more sand to the top of the plastic, level it and put down some of those 16" x 16" concrete pavers. Those are easy to keep level and any water dripping down for any reason would fall into the cracks of the pavers and be absorbed by the sand. Just a thought.