How to protect a shed floor from ground moisture

I needed some sort of workshop/storage/playhouse, so I thought I’d build a shed with three sections:

• Workshop – 2.4m x2.4m
• Storage – 1.2m x 2.4m
• Playhouse – 2.4m x 2.4m

Now I intend on turning the playhouse section into a gym, but I have a problem. After four years the OSB3 floor has gone spongy and needs replacing for something stronger to take the weight of the equipment and the duress of what it is to be used for.

I think I know a couple of contributing factors that may have led to the floor deteriorating. The end of the garden has always been soggy. Even in the summer, it takes a good bit of nice weather to dry it out and even then it’s normally only the surface that's dry. I dug up and filled the area (7m(L) x 3m(W) x 0.3m(D) with 40mm gravel and used that for the shed base to sit on.

Also, when building the shed it started to rain, so I covered it in some sticky backed plastic we had lying around (the stuff you use to protect laminate flooring when painting etc) to protect it. As this covering was on the floor, any treatment to it was overlooked and I think the plastic covering didn’t allow the OSB3 to breathe and as such it retained any natural moisture it absorbed over the years, so it went spongy.

The Issue

Now that I’ve pulled the OSB3 floor up, I intend on replacing it with some Spruce floorboards. Obviously, I’d like to not have to do this again in a few years so I’d like to take this time to do everything I’ve thought of in hindsight to protect the base, BUT, I’m not sure what to do.

After the long story, this is where you come in!

These are some of the approaches I’ve thought of, but I’m not sure what’s best:

1… Lift up the shed and lay some DPM on top of the gravel to stop moisture from rising up through the base. As much as I’d like to do this, the shed weight is well in excess of a ton so it’s not an option

2… Lay some DPM OVER the base’s bearers and lay the floorboard on top of it to protect the floorboards, but what about the base under it?

3… Paint all the base’s bearers and the underside of the floorboards in some kind of wood protection (not sure which type)

• Wood preserver?
• Bitumen?

This is pretty much all the info I can think of, so any help with what to do to help protect and preserve the base and floor would be VERY much appreciated.

I look forward to your responses if you have time.

Thanks!

UPDATE: I've added some photos for reference.

• 4×2 frame waiting to be covered in 18mm OSB3

• Mostly complete bar additional trimmings and sorting the garden

• Exposed base after pulling the floor up: THE POINT OF THE POST