^^ similar here. I have a dirt/gravel floor hay shed and had all kinds of mold issues with pallets over tarps, or tarps over pallets. Finally tacked plywood on top of pallets and it’s been fantastic for 4 years. No mold, no moisture, and no hay in the pallets.
I store some hay in my garage on the concrete floor and just set it directly on the concrete, because it’s temporary (easy access for hay delivery in winter, and I move it to the barn 20 bales at a time). But if i was going to store there for longer I might just put plywood over the concrete. If permanent, I might put pallets and then plywood.