Removing skylight from resin storage shed

I looked at your profile, but don't know what part of the country you are in. Where you are makes a lot of difference. The building you are considering is a wooden structure, fairly heavy and can hold some cargo. Any building that size deserves a decent foundation. If you don't support it right, it will warp, shift and bend with the seasons, resulting in screwed doors and separations on the siding.since you said the ground can be wet in season, I would recommended a foundation that keeps it off the ground, a vapor barrier to protect he floor and is deep enough to withstand ground heaves if you are in an area where the ground freezes in winter. If you are in a non-freeze area, I think you may want to at least support the corners and side walls with blocks at least 12 inches in the ground. Any and all water you can stop from attacking your new building will increase it's life span. Your idea of a gravel base with supports would be a minimum. Let me know where you are, and I may be able to give you better advise.

Absolutely, remove the insulation & don't install any drywall. Floor venting is a start, though not needed, but mid-level or high-level venting is best. Bleach the interior down, scrub it with a broom, hose it out & leave the door(s) open for a couple of dry days to get rid of any mold.

A shed must be free to change with the outdoor conditions as quickly as possible. Organic items should not be stored in the shed. So no, Cardboard or Interior Upholstered & Fabric. Outdoor only Cushions, Table Umbrellas & Wood Furniture are mostly fine, but should be cleaned & sunbathed annually.